Investigating sources of non-response bias in a population-based seroprevalence study of vaccine-preventable diseases in the Netherlands.

BACKGROUND: PIENTER 3 (P3), conducted in 2016/17, is the most recent of three nationwide serological surveys in the Netherlands. The surveys aim to monitor the effects of the National Immunisation Programme (NIP) by assessing population seroprevalence of included vaccine preventable diseases (VPDs). The response rate to the main sample was 15.7% (n = 4,983), following a decreasing trend in response compared to the previous two PIENTER studies (P1, 55.0%; 1995/1996 [n = 8,356] and P2, 33.0%; 2006/2007 [n = 5,834]). Non-responders to the main P3 survey were followed-up to complete a "non-response" questionnaire, an abridged 9-question version of the main survey covering demographics, health, and vaccination status. We assess P3 representativeness and potential sources of non-response bias, and trends in decreasing participation rates across all PIENTER studies. METHODS: P3 invitees were classified into survey response types: Full Participants (FP), Questionnaire Only (QO), Non-Response Questionnaire (NRQ) and Absolute Non-Responders (ANR). FP demographic and health indicator data were compared with Dutch national statistics, and then the response types were compared to each other. Random forest algorithms were used to predict response type. Finally, FPs from all three PIENTERs were compared to investigate the profile of survey participants through time. RESULTS: P3 FPs were in general healthier, younger and higher educated than the Dutch population. Random forest was not able to differentiate between FPs and ANRs, but when predicting FPs from NRQs we found evidence of healthy-responder bias. Participants of the three PIENTERs were found to be similar and are therefore comparable through time, but in line with national trends we found P3 participants were less inclined to vaccinate than previous cohorts. DISCUSSION: The PIENTER biobank is a powerful tool to monitor population-level protection against VPDs across 30 years in The Netherlands. However, future PIENTER studies should continue to focus on improving recruitment from under-represented groups, potentially by considering alternative and mixed survey modes to improve both overall and subgroup-specific response. Whilst non-responder bias is unlikely to affect seroprevalence estimates of high-coverage vaccines, the primary aim of the PIENTER biobank, other studies with varied vaccination/disease exposures should consider the influence of bias carefully.

Factors potentially contributing to the decline of the mpox outbreak in the Netherlands, 2022 and 2023.

BackgroundIn 2022 and 2023, a global outbreak of mpox affected mostly gay, bisexual and other men having sex with men (GBMSM). Outbreak control in the Netherlands included isolation, quarantine, post-exposure prophylaxis vaccination and primary preventive vaccination (PPV).AimWe describe the course of the outbreak, the vaccination programme, vaccine effectiveness (VE) of full vaccination against symptomatic disease, and trends in behaviour to generate hypotheses about factors that influenced the outbreak's decline.MethodsIn this observational study, we collected data from public health services on notified cases, number of PPV invitations and PPV doses administered. We calculated PPV uptake and coverage. Trends in behavioural data of GBMSM visiting sexual health centres were analysed for all consultations in 2022. We estimated VE using the screening method.ResultsUntil 31 December 2023, 1,294 mpox cases were reported. The outbreak peaked in early July 2022 and then declined sharply. PPV started on 25 July 2022; in total 29,851 doses were administered, 45.8% received at least one dose, 35.4% were fully vaccinated. The estimated VE was 68.2% (95% CI 4.3-89.5%). We did not observe an evident decrease in high-risk behaviour.DiscussionIt is unlikely that PPV was a driver of the outbreak's decline, as incidence started to decline well before the start of the PPV programme. The possible impact of behavioural change could not be demonstrated with the available indicators, however, the data had limitations, hampering interpretation. We hypothesise that infection-induced immunity in high-risk groups was an important factor explaining the decline.

Antibody Responses to Respiratory Syncytial Virus: A Cross-Sectional Serosurveillance Study in the Dutch Population Focusing on Infants Younger Than 2 Years.

BACKGROUND: Respiratory syncytial virus (RSV) generally causes mild disease but can cause severe infections in (premature) infants and elderly adults. Here, we studied RSV-specific antibody concentrations throughout life with emphasis on infants and chronic obstructive pulmonary disease (COPD) patients. METHODS: Sera (N = 2655) from 2 nationwide cross-sectional studies in the Netherlands including individuals aged 0-90 years were analyzed for IgG and IgA antibodies to RSV prefusion F, postfusion F, N, Ga, and Gb proteins and for antibody avidity in 42 COPD patients. RESULTS: Maternal IgG concentrations declined to age 10-12 months. After the first year of life, approximately 40% of children lacked infection-induced IgA antibodies and may therefore be uninfected. All Dutch children showed serological evidence of RSV infection by age 3 years. Antibody concentrations reached a plateau by age 5-9 years and remains constant throughout life. COPD patients had similar levels and avidity of RSV-specific IgG antibodies compared with age-matched healthy controls. CONCLUSIONS: RSV-IgG antibody patterns throughout life can be used to estimate the degree of immunity acquisition to RSV and to identify groups at increased risk of infection. Seroprevalence of IgA could be a proxy to determine RSV infection in children younger than 1 year.

Associations Between Measures of Social Distancing and Severe Acute Respiratory Syndrome Coronavirus 2 Seropositivity: A Nationwide Population-based Study in the Netherlands.

This large, nationwide, population-based, seroepidemiological study provides evidence of the effectiveness of physical distancing (>1.5 m) and indoor group size reductions in reducing severe acute respiratory syndrome coronavirus 2 infection. Additionally, young adults may play an important role in viral spread, contrary to children up until age 12 years with whom close contact is permitted. CLINICAL TRIALS REGISTRATION: NTR8473.

Impact of physical distancing measures against COVID-19 on contacts and mixing patterns: repeated cross-sectional surveys, the Netherlands, 2016-17, April 2020 and June 2020.

BackgroundDuring the COVID-19 pandemic, many countries have implemented physical distancing measures to reduce transmission of SARS-CoV-2.AimTo measure the actual reduction of contacts when physical distancing measures are implemented.MethodsA cross-sectional survey was carried out in the Netherlands in 2016-17, in which participants reported the number and age of their contacts the previous day. The survey was repeated among a subsample of the participants in April 2020, after strict physical distancing measures were implemented, and in an extended sample in June 2020, after some measures were relaxed.ResultsThe average number of community contacts per day was reduced from 14.9 (interquartile range (IQR): 4-20) in the 2016-17 survey to 3.5 (IQR: 0-4) after strict physical distancing measures were implemented, and rebounded to 8.8 (IQR: 1-10) after some measures were relaxed. All age groups restricted their community contacts to at most 5, on average, after strict physical distancing measures were implemented. In children, the number of community contacts reverted to baseline levels after measures were eased, while individuals aged 70 years and older had less than half their baseline levels.ConclusionStrict physical distancing measures greatly reduced overall contact numbers, which likely contributed to curbing the first wave of the COVID-19 epidemic in the Netherlands. However, age groups reacted differently when measures were relaxed, with children reverting to normal contact numbers and elderly individuals maintaining restricted contact numbers. These findings offer guidance for age-targeted measures in future waves of the pandemic.

Dose-Response Relationship of a Web-Based Tailored Intervention Promoting Human Papillomavirus Vaccination: Process Evaluation of a Randomized Controlled Trial.

BACKGROUND: In the Netherlands, human papillomavirus (HPV) vaccination uptake remains low. To improve informed decision making (IDM) and HPV vaccination acceptability, we systematically developed an interactive, web-based tailored intervention to which mothers of Dutch girls were invited to participate. OBJECTIVE: The aim of this study was to provide insight into the intervention's working mechanisms by evaluating (1) program use, (2) program acceptability, and (3) the relationship of program use with program acceptability and intervention effects (ie, dose-response relationship). METHODS: Only mothers from the intervention arm of a randomized controlled trial that assessed the effectiveness of the web-based, tailored intervention were included in this study. They were invited to visit the website of the web-based intervention between baseline (January 2015, just before access to the intervention) and follow up (March 2015, prior to the first HPV vaccination). Indicators for program use were time of website use (ie, duration of intervention exposure) and completeness (ie, proportion of all available web pages visited). HPV vaccination uptake registered by Praeventis was used as the primary outcome. Secondary outcomes were IDM, decisional conflict, and social-psychological determinants of HPV vaccination uptake. RESULTS: Among the 3995 invited mothers, 2509 (62.80%) logged on to the website, 2239 of whom (89.24%) visited at least one page of the intervention components. On average, mothers spent 21.39 minutes (SD 12.41) on the website and completed 50.04% (SD 26.18%) of the website components. Participants rated the website 7.64 (SD 1.39) on a 10-point scale. Program acceptability was significantly associated with completeness (ß=4.36, P<.001), but not with time of website use (ß=-.07, P=.77). Intention-to-treat analysis (N=3995) showed a significant positive effect of completeness on all outcome measures (all P<.003; Bonferroni-corrected α=.05/15 factors), including on HPV vaccination uptake. Time of website use had a significant positive effect on all outcomes (all P<.003), except for uptake (P=.20), risk perception when not vaccinated (P=.14), subjective norms (P=.03), and habit (P=.01). CONCLUSIONS: Program use and acceptability of the intervention were adequate. Completeness was positively associated with acceptability. Furthermore, positive effects (ie, dose-response effects) were found for completeness and time of website use on the mothers' IDM, decisional conflict, and almost all of the social-psychological determinants of HPV vaccination acceptability. In addition, the extent to which mothers completed the intervention had a positive impact on their daughters' vaccination uptake. This indicates that the web-based, tailored intervention fits well with the mothers' needs, and that completeness of use is essential for improving HPV vaccination uptake, acceptability, and IDM. Program use should therefore be promoted. TRIAL REGISTRATION: Netherlands Trial Register NTR4795; https://www.trialregister.nl/trial/4795.

Monitoring stance towards vaccination in twitter messages.

BACKGROUND: We developed a system to automatically classify stance towards vaccination in Twitter messages, with a focus on messages with a negative stance. Such a system makes it possible to monitor the ongoing stream of messages on social media, offering actionable insights into public hesitance with respect to vaccination. At the moment, such monitoring is done by means of regular sentiment analysis with a poor performance on detecting negative stance towards vaccination. For Dutch Twitter messages that mention vaccination-related key terms, we annotated their stance and feeling in relation to vaccination (provided that they referred to this topic). Subsequently, we used these coded data to train and test different machine learning set-ups. With the aim to best identify messages with a negative stance towards vaccination, we compared set-ups at an increasing dataset size and decreasing reliability, at an increasing number of categories to distinguish, and with different classification algorithms. RESULTS: We found that Support Vector Machines trained on a combination of strictly and laxly labeled data with a more fine-grained labeling yielded the best result, at an F1-score of 0.36 and an Area under the ROC curve of 0.66, considerably outperforming the currently used sentiment analysis that yielded an F1-score of 0.25 and an Area under the ROC curve of 0.57. We also show that the recall of our system could be optimized to 0.60 at little loss of precision. CONCLUSION: The outcomes of our study indicate that stance prediction by a computerized system only is a challenging task. Nonetheless, the model showed sufficient recall on identifying negative tweets so as to reduce the manual effort of reviewing messages. Our analysis of the data and behavior of our system suggests that an approach is needed in which the use of a larger training dataset is combined with a setting in which a human-in-the-loop provides the system with feedback on its predictions.

Third national biobank for population-based seroprevalence studies in the Netherlands, including the Caribbean Netherlands.

BACKGROUND: This paper outlines the methodology, study population and response rate of a third large Dutch population-based cross-sectional serosurvey carried-out in 2016/2017, primarily aiming to obtain insight into age-specific seroprevalence of vaccine-preventable diseases to evaluate the National Immunization Programme (NIP). In addition, Caribbean Netherlands (CN) was included, which enables additional research into tropical pathogens. METHODS: A two-stage cluster sampling technique was used to draw a sample of Dutch residents (0-89 years) (NS), including an oversampling of non-Western migrants, persons living in low vaccination coverage (LVC) areas, and an extra sample of persons born in Suriname, Aruba and the former Dutch Antilles (SAN). A separate sample was drawn for each Caribbean island. At the consultation hours, questionnaires, blood samples, oro- and nasopharyngeal swabs, faeces, - and only in the Netherlands (NL) saliva and a diary about contact patterns - were obtained from participants. Vaccination- and medical history was retrieved, and in CN anthropometric measurements were taken. RESULTS: In total, blood samples and questionnaires were collected from 9415 persons: 5745 (14.4%) in the NS (including the non-Western migrants), 1354 (19.8%) in LVC areas, 501 (6.9%) SAN, and 1815 (23.4%) in CN. CONCLUSIONS: This study will give insight into protection of the population against infectious diseases included in the NIP. Research based on this large biobank will contribute to public health (policy) in NL and CN, e.g., regarding outbreak management and emerging pathogens. Further, we will be able to extend our knowledge on infectious diseases and its changing dynamics by linking serological data to results from additional materials collected, environmental- and pharmacological data.

Motivational and contextual determinants of HPV-vaccination uptake: A longitudinal study among mothers of girls invited for the HPV-vaccination.

BACKGROUND: In the Netherlands, HPV-vaccination uptake among 12-year-old girls remains to be lower (61% in 2016) than expected. The present study is about 1) replicating the extent to which social-psychological determinants found in earlier cross-sectional studies explain HPV-vaccination intention, and 2) testing whether HPV-vaccination intention, as well as other social-psychological determinants, are good predictors of future HPV-vaccination uptake in a longitudinal design. METHODS: A random sample of mothers of girls invited for the vaccination in 2015 was drawn from the Dutch vaccination register (Praeventis) (N=36,000) and from three online panels (N=2483). Two months prior to the vaccination of girls, their mothers were requested to complete a web-based questionnaire by letter (Praeventis sample) or by e-mail (panel samples). HPV-vaccination uptake was derived from Praeventis. Backward linear and logistic regression analyses were conducted to examine most dominant predictors of HPV-vaccination intention and uptake, respectively. The total sample used for data analyses consisted of 8062 mothers. Response rates were 18% for the Praeventis sample and 47% for the panel samples. RESULTS: HPV-vaccination intention was best explained by attitude, beliefs, subjective norms, habit, and perceived relative effectiveness of the vaccination; they explained 83% of the variance in HPV-vaccination intention. Intention appeared to be the only stable predictor of HPV-vaccination uptake and explained 43% of the variance in HPV-vaccination uptake. CONCLUSIONS: These results confirm what was found by earlier cross-sectional studies, and provide strong leads for selecting relevant targets in the planning of future communication strategies aiming to improve HPV-vaccination uptake.

The intention of Dutch general practitioners to offer vaccination against pneumococcal disease, herpes zoster and pertussis to people aged 60 years and older.

BACKGROUND: Increasing life expectancy results in a larger proportion of older people susceptible to vaccine preventable diseases (VPDs). In the Netherlands, influenza vaccination is routinely offered to people aged 60 years and older. Vaccination against pneumococcal disease, herpes zoster and pertussis is rarely used. These vaccines will be evaluated by the Dutch Health Council and might be routinely offered to older people in the near future. Possible expansion of the program depends partly on the willingness of general practitioners (GPs) to endorse additional vaccinations. In this study, we assessed predictors of GPs' attitude and intention to vaccinate people aged 60 years and older. METHODS: GPs (N = 12.194) were invited to fill in an online questionnaire consisting of questions about social cognitive factors that can influence the willingness of GPs to vaccinate people aged 60 years and older, including underlying beliefs, practical considerations of adding more vaccines to the national program, demographics, and GPs' patient population characteristics. The questionnaire was filled in by 732 GPs. RESULTS: GPs were positive both about vaccination as a preventive tool and the influenza vaccination program, but somewhat less positive about expanding the current program. Prediction analysis showed that the intention of GPs to offer additional vaccination was predicted by their attitude towards offering additional vaccination, towards vaccination as a preventive tool, towards offering vaccination during an outbreak and on GPs opinion regarding suitability to offer additional vaccination (R2 = 0.60). The attitude of GPs towards offering additional vaccination was predicted by the perceived severity of herpes zoster and pneumonia, as well as the perceived incidence of herpes zoster. Severity of diseases was ranked as important argument to recommend vaccination, followed by effectiveness and health benefits of vaccines. CONCLUSION: Providing GPs with evidence-based information about the severity and prevalence of diseases, and effectiveness and health benefits of the vaccines, together with an active role of GPs in informing older people about vaccines, could modify the intention towards additional vaccination of people 60 years and older.

Consistency between stated and revealed preferences: a discrete choice experiment and a behavioural experiment on vaccination behaviour compared.

BACKGROUND: Discrete Choice Experiments (DCEs) are increasingly used in studies in healthcare research but there is still little empirical evidence for the predictive value of these hypothetical situations in similar real life circumstances. The aim of this paper is to compare the stated preferences in a DCE and the accompanying questionnaire with the revealed preferences of young parents who have to decide whether to vaccinate their new born child against hepatitis B. METHODS: A DCE asking parents to decide in which scenario they would be more inclined to vaccinate their child against hepatitis B. The stated preference was estimated by comparing the per respondent utility of the most realistic scenario in which parents could choose to vaccinate their child against hepatitis B, with the utility of the opt-out, based on the mixed logit model from the DCE. This stated preference was compared with the actual behaviour of the parents concerning the vaccination of their new born child. RESULTS: In 80% of the respondents the stated and revealed preferences corresponded. The positive predictive value is 85% but the negative predictive value is 26%. CONCLUSIONS: The predictive value of the DCE in this study is satisfactory for predicting the positive choice but not for predicting the negative choice. However, the behaviour in this study is exceptional in the sense that most people chose to vaccinate. Future studies should focus on behaviours with a larger variance in the population.

Vaccination decision-making of immigrant parents in the Netherlands; a focus group study.

BACKGROUND: Although the vaccination coverage in most high income countries is high, variations in coverage rates on the national level among different ethnic backgrounds are reported. A qualitative study was performed to explore factors that influence decision-making among parents with different ethnic backgrounds in the Netherlands. METHODS: Six focus groups were conducted with 33 mothers of Moroccan, Turkish and other ethnic backgrounds with at least one child aged 0-4 years. Data were analysed using thematic analysis. RESULTS: Parents had a positive attitude towards childhood vaccination and a high confidence in the advices of Child Vaccine Providers (CVPs). Vaccinating their children was perceived as self-evident and important. Parents do perceive a language barrier in understanding the provided NIP-information, and they had a need for more NIP- information, particularly about the targeted diseases. Another barrier parents perceived was the distance to the Child Welfare Center (CWC), especially when the weather was bad and when they had no access to a car. CONCLUSION: More information about targeted diseases and complete information regarding benefits and drawbacks of the NIP should be provided to the parents. To fulfill parents' information needs, NIP information meetings can be organized at CWCs in different languages. Providing NIP information material in Turkish, Arabic and Berber language with easy access is also recommended. Providing information tailored to these parents' needs is important to sustain high vaccination participation, and to ensure acceptance of future vaccinations.

Disease detection or public opinion reflection? Content analysis of tweets, other social media, and online newspapers during the measles outbreak in The Netherlands in 2013.

BACKGROUND: In May 2013, a measles outbreak began in the Netherlands among Orthodox Protestants who often refuse vaccination for religious reasons. OBJECTIVE: Our aim was to compare the number of messages expressed on Twitter and other social media during the measles outbreak with the number of online news articles and the number of reported measles cases to answer the question if and when social media reflect public opinion patterns versus disease patterns. METHODS: We analyzed measles-related tweets, other social media messages, and online newspaper articles over a 7-month period (April 15 to November 11, 2013) with regard to topic and sentiment. Thematic analysis was used to structure and analyze the topics. RESULTS: There was a stronger correlation between the weekly number of social media messages and the weekly number of online news articles (P<.001 for both tweets and other social media messages) than between the weekly number of social media messages and the weekly number of reported measles cases (P=.003 and P=.048 for tweets and other social media messages, respectively), especially after the summer break. All data sources showed 3 large peaks, possibly triggered by announcements about the measles outbreak by the Dutch National Institute for Public Health and the Environment and statements made by well-known politicians. Most messages informed the public about the measles outbreak (ie, about the number of measles cases) (93/165, 56.4%) followed by messages about preventive measures taken to control the measles spread (47/132, 35.6%). The leading opinion expressed was frustration regarding people who do not vaccinate because of religious reasons (42/88, 48%). CONCLUSIONS: The monitoring of online (social) media might be useful for improving communication policies aiming to preserve vaccination acceptability among the general public. Data extracted from online (social) media provide insight into the opinions that are at a certain moment salient among the public, which enables public health institutes to respond immediately and appropriately to those public concerns. More research is required to develop an automatic coding system that captures content and user's characteristics that are most relevant to the diseases within the National Immunization Program and related public health events and can inform official responses.

Waning of maternal antibodies against measles, mumps, rubella, and varicella in communities with contrasting vaccination coverage.

BACKGROUND: The combined measles, mumps, and rubella (MMR) vaccine has been successfully administered for >20 years. Because of this, protection by maternal antibodies in infants born to vaccinated mothers might be negatively affected. METHODS: A large cross-sectional serologic survey was conducted in the Netherlands during 2006-2007. We compared the kinetics of antibody concentrations in children and women of childbearing age in the highly vaccinated general population with those in orthodox Protestant communities that were exposed to outbreaks. RESULTS: The estimated duration of protection by maternal antibodies among infants in the general population, most of whom were born to vaccinated mothers, was short: 3.3 months for measles, 2.7 months for mumps, 3.9 months for rubella, and 3.4 months for varicella. The duration of protection against measles was 2 months longer for infants born in the orthodox communities, most of whom had unvaccinated mothers. For rubella, mothers in the orthodox communities had higher concentrations of antibodies as compared to the general population. CONCLUSION: Children of mothers vaccinated against measles and, possibly, rubella have lower concentrations of maternal antibodies and lose protection by maternal antibodies at an earlier age than children of mothers in communities that oppose vaccination. This increases the risk of disease transmission in highly vaccinated populations.

Why parents refuse childhood vaccination: a qualitative study using online focus groups.

BACKGROUND: In high income countries, vaccine-preventable diseases have been greatly reduced through routine vaccination programs. Despite this success, many parents question, and a small proportion even refuse vaccination for their children. As no qualitative studies have explored the factors behind these decisions among Dutch parents, we performed a study using online focus groups. METHODS: In total, eight online focus groups (n = 60) which included Dutch parents with at least one child, aged 0-4 years, for whom they refused all or part of the vaccinations within the National Immunization Program (NIP). A thematic analysis was performed to explore factors that influenced the parents' decisions to refuse vaccination. RESULTS: Refusal of vaccination was found to reflect multiple factors including family lifestyle; perceptions about the child's body and immune system; perceived risks of disease, vaccine efficacy, and side effects; perceived advantages of experiencing the disease; prior negative experience with vaccination; and social environment. The use of online focus groups proved to be an effective qualitative research method providing meaningful data. CONCLUSION: Information provided by the NIP turned out to be insufficient for this group of parents. More trust in the NIP and deliberate decisions might result from increased parental understanding of lifestyle and disease susceptibility, the impact of vaccinations on the immune system, and the relative risks of diseases and their vaccines. The public health institute should also inform parents that the NIP is recommended but non-mandatory.

Parental information-seeking behaviour in childhood vaccinations.

BACKGROUND: People want to be well informed and ask for more information regarding their health. The public can use different sources (i.e. the Internet, health care providers, friends, family, television, radio, and newspapers) to access information about their health. Insight into the types and sources of vaccine related information that parents use, and reasons why they seek extra information is needed to improve the existing information supply about childhood vaccinations. METHODS: Dutch parents with one or more children aged 0-4 years received an online questionnaire (N=4,000) measuring psychosocial determinants of information-seeking behaviour and self-reports of types and sources of vaccine information searched for (response rate 14.8%). We also tested two invitation approaches (i.e., reply card versus Internet link in invitation letter) to observe the difference in response rate. RESULTS: Almost half of the parents (45.8%) searched for extra information. Of all the respondents, 13% indicated they had missed some information, particularly about side effects of vaccines (25%). Intention to search for vaccination information was influenced by positive attitude and perceived social norm towards information-seeking behaviour. There was no difference in the response rate between the two invitation approaches. CONCLUSIONS: The information provided by the National Immunization Programme (NIP) might be sufficient for most parents. However, some parents mentioned that they did not receive enough information about side effects of vaccinations, which was also the topic most searched for by parents. Public Health Institutes (PHIs) and child healthcare workers should therefore be aware of the importance to mention this aspect in their communication (materials) towards parents. The PHIs must ensure that their website is easy to find with different search strategies. Since the child healthcare worker is perceived as the most reliable information source, they should be aware of their role in educating parents about the NIP.

An exploratory qualitative assessment of factors influencing childhood vaccine providers' intention to recommend immunization in the Netherlands.

BACKGROUND: Under the Dutch national immunization program (NIP), childhood vaccination is not mandatory, but its recommendation by childhood vaccine providers (CVP) is important for maintaining high vaccination coverage. We therefore examined factors related to providers' intentions to recommend vaccinations to parents of young children. METHODS: We conducted four focus group discussions with nurses and physicians who provide vaccines to children 0-4 years old in diverse regions of the Netherlands. Three groups represented CVPs at child welfare centers (CWCs) serving the general population, with the fourth representing anthroposophical CWCs. Elements of the Theory of Planned Behaviour (TPB) were used to design the groups; thematic analysis was used to structure and analyze the dataset. RESULTS: Four main themes emerged, including 1) perceived responsibility: to promote vaccines and discuss pros and cons with parents (although this was usually not done if parents readily accepted the vaccination); 2) attitudes toward the NIP: mainly positive, but doubts as to NIP plans to vaccinate against diseases with a low perceived burden; 3) organizational factors: limited time and information can hamper discussions with parents; 4) relationship with parents: crucial and based mainly on communication to establish trust. Compared to CVPs at standard CWCs, the anthroposophical CWCs spent more time communicating and were more willing to adapt the NIP to individual cases. CONCLUSIONS: Our qualitative assessment provides an overview of beliefs associated with providers' intention to recommend vaccinations. They were motivated to support the NIP, but their intentions to recommend vaccinations were affected by the perceived relevance of the vaccines, practical issues like limited time and by certain types of resistant parents. These results will inform future studies to test the magnitude and relative impact of these factors.

Participation in and attitude towards the national immunization program in the Netherlands: data from population-based questionnaires.

BACKGROUND: Knowledge about the determinants of participation and attitude towards the National Immunisation Program (NIP) may be helpful in tailoring information campaigns for this program. Our aim was to determine which factors were associated with nonparticipation in the NIP and which ones were associated with parents' intention to accept remaining vaccinations. Further, we analyzed possible changes in opinion on vaccination over a 10 year period. METHODS: We used questionnaire data from two independent, population-based, cross-sectional surveys performed in 1995-96 and 2006-07. For the 2006-07 survey, logistic regression modelling was used to evaluate what factors were associated with nonparticipation and with parents' intention to accept remaining vaccinations. We used multivariate multinomial logistic regression modelling to compare the results between the two surveys. RESULTS: Ninety-five percent of parents reported that they or their child (had) participated in the NIP. Similarly, 95% reported they intended to accept remaining vaccinations. Ethnicity, religion, income, educational level and anthroposophic beliefs were important determinants of nonparticipation in the NIP. Parental concerns that played a role in whether or not they would accept remaining vaccinations included safety of vaccinations, maximum number of injections, whether vaccinations protect the health of one's child and whether vaccinating healthy children is necessary. Although about 90% reported their opinion towards vaccination had not changed, a larger proportion of participants reported to be less inclined to accept vaccination in 2006-07 than in 1995-96. CONCLUSION: Most participants had a positive attitude towards vaccination, although some had doubts. Groups with a lower income or educational level or of non-Western descent participated less in the NIP than those with a high income or educational level or indigenous Dutch and have been less well identified previously. Particular attention ought to be given to these groups as they contribute in large measure to the rate of nonparticipation in the NIP, i.e., to a greater extent than well-known vaccine refusers such as specific religious groups and anthroposophics. Our finding that the proportion of the population inclined to accept vaccinations is smaller than it was 10 years ago highlights the need to increase knowledge about attitudes and beliefs regarding the NIP.

Population-based serology reveals risk factors for RSV infection in children younger than 5 years.

Respiratory syncytial virus (RSV) infection is a leading cause of hospitalization in infants. Underlying risk factors for RSV infection in the general population are not well understood, as previous work has focused on severe outcomes of infection in a clinical setting. Here we use RSV-specific IgG and IgA antibody measurements from two population-based cross-sectional serosurveys carried out in the Netherlands (n = 682) to classify children up to 5 years as seronegative or seropositive. We employ a generalized additive model to estimate the probability of prior RSV infection as function of age, date of birth within the year, and other risk factors. The analyses show that the majority of children have experienced a RSV infection before the age of 2 years. Age and birthdate are strong predictors of RSV infection in the first years of life, and children born in summer have higher estimated probability of infection than those born in winter [e.g., 0.56 (95% CI 0.45-0.66) vs. 0.32 (0.21-0.45) at age 1 year]. Our analyses reveal that the mean age at infection depends on date of birth, which has implications for the design of vaccination programmes and prioritisation schemes for the prophylactic use of monoclonal antibodies.

Age-specific HPV seroprevalence among young females in The Netherlands.

OBJECTIVES: To obtain insight into the age-specific seroprevalence for human papillomavirus (HPV) 6, 11, 16 and 18 among females before introduction of HPV vaccination in The Netherlands. METHODS: In a population-based study in The Netherlands, 637 sera of 11-26-year-old females were tested for HPV6/11/16/18 antibodies. Sera were tested using a competitive Luminex assay with neutralising monoclonal antibodies specific for each serotype. Associations between HPV seropositivity, demographics, and sexual behaviour were studied with logistic regression. Seroprevalences were standardised for age and urbanisation degree to the general female population in The Netherlands. RESULTS: The overall prevalence of antibodies against HPV6/11/16/18 was 7.9%. 4.3% had antibodies against HPV types 6/11, and 4.4% had antibodies against HPV types 16/18. HPV seropositivity significantly increased with age (OR 1.2; 95% CI 1.1 to 1.4), starting at the age of 16 years (median age of sexual debut in The Netherlands). A former diagnosis with sexually transmitted infections was also significantly associated with HPV seropositivity (OR 6.3; 95% CI 2.2 to 17.9). CONCLUSIONS: In addition to 12-year-old girls who are targeted for routine HPV vaccination, also girls up to 16 years are likely to benefit substantially from HPV16/18 vaccination. Testing for the presence of HPV antibodies in females after introduction of vaccination makes it possible to monitor the impact of immunisation at the population level.