Increase in invasive group A streptococcal infections (iGAS) in children and older adults, Norway, 2022 to 2024.

At the end of 2022 and most notably during the first half of 2023, the number of invasive group A streptococcus (iGAS) notifications increased in Norway, largely affecting children younger than 10 years, as observed in several other countries. Following this atypical season, a new surge in the number of iGAS notifications began in December 2023 and peaked between January and February 2024, now particularly affecting both children younger than 10 years and older adults (70 years and above).

Effectiveness of one and two doses of acellular pertussis vaccines against laboratory-confirmed pertussis requiring hospitalisation in infants: Results of the PERTINENT sentinel surveillance system in six EU/EEA countries, December 2015 - December 2019.

BACKGROUND: Monitoring effectiveness of pertussis vaccines is necessary to adapt vaccination strategies. PERTINENT, Pertussis in Infants European Network, is an active sentinel surveillance system implemented in 35 hospitals across six EU/EEA countries. We aim to measure pertussis vaccines effectiveness (VE) by dose against hospitalisation in infants aged <1 year. METHODS: From December 2015 to December 2019, participating hospitals recruited all infants with pertussis-like symptoms. Cases were vaccine-eligible infants testing positive for Bordetella pertussis by PCR or culture; controls were those testing negative to all Bordetella spp. For each vaccine dose, we defined an infant as vaccinated if she/he received the corresponding dose >14 days before symptoms. Unvaccinated were those who did not receive any dose. We calculated (one-stage model) pooled VE as 100*(1-odds ratio of vaccination) adjusted for country, onset date (in 3-month categories) and age-group (when sample allowed it). RESULTS: Of 1,393 infants eligible for vaccination, we included 259 cases and 746 controls. Median age was 16 weeks for cases and 19 weeks for controls (p < 0.001). Median birth weight and gestational age were 3,235 g and week 39 for cases, 3,113 g and week 39 for controls. Among cases, 119 (46 %) were vaccinated: 74 with one dose, 37 two doses, 8 three doses. Among controls, 469 (63 %) were vaccinated: 233 with one dose, 206 two doses, 30 three doses. Adjusted VE after at least one dose was 59 % (95 %CI: 36-73). Adjusted VE was 48 % (95 %CI: 5-71) for dose one (416 eligible infants) and 76 % (95 %CI: 43-90) for dose two (258 eligible infants). Only 42 infants were eligible for the third dose. CONCLUSIONS: Our results suggest moderate one-dose and two-dose VE in infants. Larger sample size would allow more precise estimates for dose one, two and three.

No association between disease severity and respiratory syncytial virus subtypes RSV-A and RSV-B in hospitalized young children in Norway.

OBJECTIVE: There is conflicting evidence whether subtypes of Respiratory syncytial virus have different seasonality or are differentially associated with clinical severity. We aimed to explore the associations between disease severity and RSV subtypes RSV-A and RSV-B and to describe the circulation of RSV subtypes pattern by season and age. METHODS: Active prospective hospital surveillance for RSV-A and RSV-B in children <59 months of age was conducted during 2015-2018. All febrile children 12-59 months of age were enrolled, whereas children <12 months were eligible if presenting with fever or respiratory symptoms. Risk factors and upper and lower respiratory tract infection was identified by linkage to national registry data and analyzed using penalized maximum likelihood logistic regression. RESULTS: Both RSV-A and B were found to co-circulate throughout all three study seasons, and no clear seasonal pattern was identified. Likewise, we found no association between sex or measures of severity with RSV-A or RSV-B. There was significantly more RSV-A than RSV-B among children with comorbidities. CONCLUSIONS: No association was found between disease severity or sex and RSV subtypes RSV-A and RSV-B in hospitalized young children in Norway.

Frequency and risk of SARS-CoV-2 reinfections in Norway: a nation-wide study, February 2020 to January 2022.

BACKGROUND: SARS-CoV-2 reinfection rates have been shown to vary depending on the circulating variant, vaccination status and background immunity, as well as the time interval used to identify reinfections. This study describes the frequency of SARS-CoV-2 reinfections in Norway using different time intervals and assesses potential factors that could impact the risk of reinfections during the different variant waves. METHODS: We used linked individual-level data from national registries to conduct a retrospective cohort study including all cases with a positive test for SARS-CoV-2 from February 2020 to January 2022. Time intervals of 30, 60, 90 or 180 days between positive tests were used to define potential reinfections. A multivariable Cox regression model was used to assess the risk of reinfection in terms of variants adjusting for vaccination status, demographic factors, and underlying comorbidities. RESULTS: The reinfection rate varied between 0.2%, 0.6% and 5.9% during the Alpha, Delta and early Omicron waves, respectively. In the multivariable model, younger age groups were associated with a higher risk of reinfection compared to older age groups, whereas vaccination was associated with protection against reinfection. Moreover, the risk of reinfection followed a pattern similar to risk of first infection. Individuals infected early in the pandemic had higher risk of reinfection than individuals infected in more recent waves. CONCLUSIONS: Reinfections increased markedly during the Omicron wave. Younger individuals, and primary infections during earlier waves were associated with an increased reinfection risk compared to primary infections during more recent waves, whereas vaccination was a protective factor. Our results highlight the importance of age and post infection waning immunity and are relevant when evaluating vaccination polices.

Estimation of the Number of Respiratory Syncytial Virus-Associated Hospitalizations in Adults in the European Union.

BACKGROUND: Respiratory syncytial virus (RSV) is a major cause of lower respiratory tract infections in adults that can result in hospitalizations. Estimating RSV-associated hospitalization is critical for planning RSV-related healthcare across Europe. METHODS: We gathered RSV-associated hospitalization estimates from the RSV Consortium in Europe (RESCEU) for adults in Denmark, England, Finland, Norway, Netherlands, and Scotland from 2006 to 2017. We extrapolated these estimates to 28 European Union (EU) countries using nearest-neighbor matching, multiple imputations, and 2 sets of 10 indicators. RESULTS: On average, 158 229 (95% confidence interval [CI], 140 865-175 592) RSV-associated hospitalizations occur annually among adults in the EU (≥18 years); 92% of these hospitalizations occur in adults ≥65 years. Among 75-84 years, the annual average is estimated at 74 519 (95% CI, 69 923-79 115) at a rate of 2.24 (95% CI, 2.10-2.38) per 1000. Among ≥85 years, the annual average is estimated at 37 904 (95% CI, 32 444-43 363) at a rate of 2.99 (95% CI, 2.56-3.42). CONCLUSIONS: Our estimates of RSV-associated hospitalizations in adults are the first analysis integrating available data to provide the disease burden across the EU. Importantly, for a condition considered in the past to be primarily a disease of young children, the average annual hospitalization estimate in adults was lower but of a similar magnitude to the estimate in young children (0-4 years): 158 229 (95% CI, 140 865-175 592) versus 245 244 (95% CI, 224 688-265 799).

Stroke After SARS-CoV-2 mRNA Vaccine: A Nationwide Registry Study.

BACKGROUND: Whether the SARS-CoV-2 mRNA vaccines may cause a transient increased stroke risk is uncertain. METHODS: In a registry-based cohort of all adult residents at December 27, 2020, in Norway, we linked individual-level data on COVID-19 vaccination, positive SARS-CoV-2 test, hospital admissions, cause of death, health care worker status, and nursing home resident status extracted from the Emergency Preparedness Register for COVID-19 in Norway. The cohort was followed for incident intracerebral bleeding, ischemic stroke, and subarachnoid hemorrhage within the first 28 days after the first/second or third dose of mRNA vaccination until January 24, 2022. Stroke risk after vaccination relative to time not exposed to vaccination was assessed by Cox proportional hazard ratio, adjusted for age, sex, risk groups, health care personnel, and nursing home resident. RESULTS: The cohort included 4 139 888 people, 49.8% women, and 6.7% were ≥80 years of age. During the first 28 days after an mRNA vaccine, 2104 people experienced a stroke (82% ischemic stroke, 13% intracerebral hemorrhage, and 5% subarachnoid hemorrhage). Adjusted hazard ratios (95% CI) after the first/second and after the third mRNA vaccine doses were 0.92 (0.85-1.00) and 0.89 (0.73-1.08) for ischemic stroke, 0.81 (0.67-0.98) and 1.05 (0.64-1.71) for intracerebral hemorrhage, and 0.64 (0.46-0.87) and 1.12 (0.57-2.19) for subarachnoid hemorrhage, respectively. CONCLUSIONS: We did not find increased risk of stroke during the first 28 days after an mRNA SARS-CoV-2 vaccine.

Effectiveness of BNT162b2 vaccine against SARS-CoV-2 Delta and Omicron infection in adolescents, Norway, August 2021 to January 2022.

OBJECTIVES: We estimated the BNT162b2 vaccine effectiveness (VE) against any (symptomatic or not) SARS-CoV-2 Delta and Omicron infection among adolescents (aged 12-17 years) in Norway from August 2021 to January 2022. METHODS: We used Cox proportional hazard models, where vaccine status was included as a time-varying covariate and models were adjusted for age, sex, comorbidities, residence county, birth country, and living conditions. RESULTS: The VE against Delta infection peaked at 68% (95% confidence interval [CI]: 64-71%) and 62% (95% CI: 57-66%) in days 21-48 after the first dose among those aged 12-15 years and 16-17 years, respectively. Among those aged 16-17 years who received two doses, the VE against Delta infection peaked at 93% (95% CI: 90-95%) in days 35-62 and decreased to 84% (95% CI: 76-89%) in ≥63 days after vaccination. We did not observe a protective effect against Omicron infection after receiving one dose. Among those aged 16-17 years, the VE against Omicron infection peaked at 53% (95% CI: 43-62%) in 7-34 days after the second dose and decreased to 23% (95% CI: 3-40%) in ≥63 days after vaccination. CONCLUSION: We found a reduced protection after two BNT162b2 vaccine doses against any Omicron infection compared to Delta. Effectiveness decreased with time from vaccination for both variants. The impact of vaccination among adolescents on reducing infection and thus transmission is limited during the Omicron dominance.

Short-term safety of COVID-19 mRNA vaccines with respect to all-cause mortality in the older population in Norway.

BACKGROUND: There have been concerns about COVID-19 vaccination safety among frail older individuals. We investigated the relationship between COVID-19 mRNA vaccination and mortality among individuals aged ≥ 70 years and whether mortality varies across four groups of health services used. METHODS: In this nationwide cohort study, we included 688,152 individuals aged ≥ 70 years at the start of the Norwegian vaccination campaign (December 27, 2020). We collected individual-level data from theNorwegian Emergency Preparedness Register for COVID-19. Vaccinated and unvaccinated individuals were matched (1:1 ratio) on the date of vaccination based on sociodemographic and clinical characteristics. The main outcome was all-cause mortality during 21 days after first dose of COVID-19 mRNA vaccination. Kaplan-Meier survival functions were estimated for the vaccinated and unvaccinated groups. We used Cox proportional-hazards regression to estimate hazard ratios (HRs) of death between vaccinated and unvaccinated individuals, with associated 95% confidence intervals (CIs), overall and by use of health services (none, home-based, short- and long-term nursing homes) and age group. RESULTS: Between December 27, 2020, and March 31, 2021, 420,771 older individuals (61.1%) were vaccinated against COVID-19. The Kaplan-Meier estimates based on the matched study sample showed a small absolute risk difference in all-cause mortality between vaccinated and unvaccinated individuals, with a lower mortality in the vaccinated group (overall HR 0.28 [95% CI: 0.24-0.31]). Similar results were obtained in analyses stratified by use of health services and age group. CONCLUSION: We found no evidence of increased short-term mortality among vaccinated individuals in the older population after matching on sociodemographic and clinical characteristics affecting vaccination and mortality.

Risk factors associated with severe disease in respiratory syncytial virus infected children under 5 years of age.

Objective: To evaluate risk factors for severe disease in children under 59 months of age hospitalized with respiratory syncytial virus (RSV) infection. Study design: We prospectively enrolled 1,096 cases of laboratory confirmed RSV infection during three consecutive RSV seasons in 2015-2018. Potential risk factors for severe disease were retrieved through patient questionnaires and linkage to national health registries. Need for respiratory support (invasive ventilation, bi-level positive airway pressure, or continuous positive airway pressure), and length of stay exceeding 72 h were used as measures of disease severity. Associations were investigated using multivariable logistic regression analyses. Multiple imputation was used to avoid bias and inference induced by missing data. Results: Risk factors associated with a need for respiratory support included age younger than 3 months of age [aOR: 6.73 (95% CI 2.71-16.7)], having siblings [aOR: 1.65 (95% CI 1.05-2.59)] and comorbidity [aOR: 2.40 (95% CI 1.35-4.24)]. The length of hospital stay >72 h was significantly associated with being younger than 3 months of age [aOR: 3.52 (95% CI 1.65-7.54)], having siblings [aOR: 1.45 (95% CI 1.01-2.08)], and comorbidity [aOR: 2.18 (95% CI 1.31-3.61)]. Sub-group analysis of children younger than 6 months of age confirmed the association between both young age and having siblings and the need for respiratory support. Conclusion: In a large cohort of children <59 months hospitalized with RSV infection, young age, comorbidity, and having siblings were associated with more severe disease.

The burden of hospital-attended influenza in Norwegian children.

Background: Norwegian health authorities do not recommend universal pediatric vaccination against seasonal influenza. We aimed to estimate the incidence of influenza by age and underlying medical conditions in hospitalized Norwegian children aged <18 years. Methods: Active surveillance for influenza in children <18 years was implemented in five hospitals during 2015-18. Children with respiratory symptoms and/or fever were prospectively enrolled and tested for influenza. Surveillance data were linked to health registry data to estimate the national burden of influenza in hospitals. Results: In 309 (10%) out of 3,010 hospital contacts, the child tested positive for influenza, corresponding to an average incidence of 0.96 hospital-attended influenza cases per 1,000 children <18 years of age. Children <1 year of age (3.8 per 1,000 children) and children with underlying medical conditions (17 per 1,000 children with bronchopulmonary dysplasia) had the highest average incidence. Among <1 year old children, 3% tested positive for influenza, compared to 25% for children aged 6-17. Few children were vaccinated against influenza. Conclusions: Children <1 year of age and children with underlying medical conditions had a higher incidence of influenza requiring hospital treatment compared to the general population. Effective interventions against seasonal influenza for children in Norway should be considered.

COVID-19 Hospitalization Among Children <18 Years by Variant Wave in Norway.

OBJECTIVES: There is limited evidence on whether the relative severity of coronavirus disease 2019 (COVID-19) in children and adolescents differs for different severe acute respiratory syndrome coronavirus 2 variants. We compare the risk of hospitalization to acute COVID-19 or multisystem inflammatory syndrome in children (MIS-C) among unvaccinated persons <18 years with COVID-19 (cases) between waves of the Alpha, Delta, and Omicron (sublineage BA.1) variants in Norway. METHODS: We used linked individual-level data from national registries to calculate adjusted risk ratios (aRR) with 95% confidence interval (CI) using multivariable log-binomial regression. We adjusted for variant wave, demographic characteristics, and underlying comorbidities. RESULTS: We included 10 538 Alpha (21 hospitalized with acute COVID-19, 7 MIS-C), 42 362 Delta (28 acute COVID-19, 14 MIS-C), and 82 907 Omicron wave cases (48 acute COVID-19, 7 MIS-C). The risk of hospitalization with acute COVID-19 was lower in the Delta (aRR: 0.53, 95% CI: 0.30-0.93) and Omicron wave (aRR: 0.40, 95% CI: 0.24-0.68), compared to the Alpha wave. We found no difference in this risk for Omicron compared to Delta. The risk of MIS-C was lower for Omicron, compared to Alpha (aRR: 0.09, 95% CI: 0.03-0.27) and Delta (aRR: 0.26, 95% CI: 0.10-0.63). CONCLUSIONS: We do not find clear evidence that different variants have influenced the risk of hospitalization with acute COVID-19 among unvaccinated children and adolescents in Norway. The lower risk of this outcome with Omicron and Delta may reflect changes in other factors over time, such as the testing strategy, maternal vaccination and/or hospitalization criteria. The emergence of Omicron has reduced the risk of MIS-C.

Milder disease trajectory among COVID-19 patients hospitalised with the SARS-CoV-2 Omicron variant compared with the Delta variant in Norway.

Using individual-level national registry data, we conducted a cohort study to estimate differences in the length of hospital stay, and risk of admission to an intensive care unit and in-hospital death among patients infected with the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) Omicron variant, compared with patients infected with Delta variant in Norway. We included 409 (38%) patients infected with Omicron and 666 (62%) infected with Delta who were hospitalised with coronavirus disease 2019 (COVID-19) as the main cause of hospitalisation between 6 December 2021 and 6 February 2022. Omicron patients had a 48% lower risk of intensive care admission (adjusted hazard ratios (aHR): 0.52, 95% confidence interval (CI): 0.34-0.80) and a 56% lower risk of in-hospital death (aHR: 0.44, 95%CI: 0.24-0.79) compared with Delta patients. Omicron patients had a shorter length of stay (with or without ICU stay) compared with Delta patients in the age groups from 18 to 79 years and those who had at least completed their primary vaccination. This supports growing evidence of reduced disease severity among hospitalised Omicron patients compared with Delta patients.

Age-Specific Estimates of Respiratory Syncytial Virus-Associated Hospitalizations in 6 European Countries: A Time Series Analysis.

BACKGROUND: Knowledge on age-specific hospitalizations associated with RSV infection is limited due to limited testing, especially in older children and adults in whom RSV infections are not expected to be severe. Burden estimates based on RSV coding of hospital admissions are known to underestimate the burden of RSV. We aimed to provide robust and reliable age-specific burden estimates of RSV-associated hospital admissions based on data on respiratory infections from national health registers and laboratory-confirmed cases of RSV. METHODS: We conducted multiseason regression analysis of weekly hospitalizations with respiratory infection and weekly laboratory-confirmed cases of RSV and influenza as covariates, based on national health registers and laboratory databases across 6 European countries. The burden of RSV-associated hospitalizations was estimated by age group, clinical diagnosis, and presence of underlying medical conditions. RESULTS: Across the 6 European countries, hospitalizations of children with respiratory infections were clearly associated with RSV, with associated proportions ranging from 28% to 60% in children younger than 3 months and we found substantial proportions of admissions to hospital with respiratory infections associated with RSV in children younger than 3 years. Associated proportions were highest among hospitalizations with ICD-10 codes of "bronchitis and bronchiolitis." In all 6 countries, annual incidence of RSV-associated hospitalizations was >40 per 1000 persons in the age group 0-2 months. In age group 1-2 years the incidence rate ranged from 1.3 to 10.5 hospitalizations per 1000. Adults older than 85 years had hospitalizations with respiratory infection associated to RSV in all 6 countries although incidence rates were low. CONCLUSIONS: Our findings highlight the substantial proportion of RSV infections among hospital admissions across different ages and may help public health professionals and policy makers when planning prevention and control strategies. In addition, our findings provide valuable insights for health care professionals attending to both children and adults presenting with symptoms of viral respiratory infections.

Pertussis epidemiology including direct and indirect effects of the childhood pertussis booster vaccinations, Norway, 1998-2019.

BACKGROUND: The acellular pertussis vaccine has been used in the Norwegian national immunisation program since 1998. Following an increase in pertussis incidence in all age groups, booster doses were introduced for 7-8-year-olds in 2006, and for 15-16-year-olds in 2013. We assessed the effects of the booster doses on pertussis incidence in different age groups to inform potential changes in vaccination policy. METHODS: We included all pertussis cases notified to the Norwegian Surveillance System for Communicable Diseases in 1998-2019. We calculated annual incidence rates (IR, per 100,000 inhabitants) by age group. We estimated average annual changes in IRs (incidence rate ratios, IRR) for each age group for 2006-2012 and 2013-2019 using Poisson regression. RESULTS: In 1998-2019, 74,675 cases of pertussis were notified. Coinciding with booster introduction, between 2006 and 2012 the IR decreased among 8-15-year-olds (from 433 to 199/100,000, IRR 0.89 [95% confidence interval 0.88-0.90]). A similar decrease was seen between 2013 and 2019 among 16-19-year-olds (from 171 to 77/100,000, IRR 0.84 [0.82-0.86]). There was no significant change in IRs among children < 1 year of age between 2006 and 2012 (IRR 0.99 [0.95-1.04]) or 2013-2019 (IRR 0.96 [0.91-1.02]). The IR decreased in both periods among adults aged 20-39 and 40+ (IRR 0.94 [0.93-0.95] and 0.92 [0.91-0.92] in 2006-2012; IRR 0.97 [0.96-0.99] and 0.97 [0.96-0.99] in 2013-2019, respectively). Despite steady, high vaccination coverage, in 2013-2019, there was an increase in the IR among children aged 1-7 (63 to 86/100,000, IRR 1.05 [1.03-1.07]) and 8-15 years (88 to 122/100,000, IRR 1.08 [1.06-1.10]). CONCLUSIONS: Pertussis booster doses have offered direct protection in the targeted age groups. Our findings suggest indirect protection in adults, while the incidence in infants hasn't changed. The recent increase in IRs among 1-15-year-olds warrants close monitoring and further evaluation of the vaccination schedule.

A Systematic Review of European Clinical Practice Guidelines for Respiratory Syncytial Virus Prophylaxis.

BACKGROUND: Since the widespread adoption of palivizumab prophylaxis in Europe, there have been a number of clinical practice guidelines (CPGs) published for the prevention of respiratory syncytial virus (RSV) infection in children. The aim of this systematic review was to identify CPGs for the prevention of RSV infection across Europe. METHODS: We performed a systematic literature search and contacted European influenza and respiratory virus networks and public health institutions, to identify national CPGs for the prevention of RSV infection. The Reporting Items for practice Guidelines in Healthcare (RIGHT) Statement checklist was applied to extract data and review the quality of reporting. RESULTS: A total of 20 national CPGs were identified, all published between 2000 and 2018. The greatest discrepancy between guidelines was the recommendations for palivizumab prophylaxis for premature infants, with recommendations varying by gestational age. All guidelines recommended or considered the use of palivizumab in infants with bronchopulmonary dysplasia, 85% (n = 17) in children with congenital heart disease (CHD), and 60% (n = 12) in children with severe combined immunodeficiency. CONCLUSIONS: We recommend that agencies publishing RSV prevention guidelines adopt the RIGHT reporting requirements when updating these guidelines to improve the presentation of the evidence-base for decisions.

Respiratory Syncytial Virus-Associated Hospital Admissions and Bed Days in Children <5 Years of Age in 7 European Countries.

BACKGROUND: Respiratory syncytial virus (RSV) is a leading cause of respiratory tract infections (RTIs) in young children. High-quality country-specific estimates of bed days and length of stay (LOS) show the population burden of RSV-RTI on secondary care services and the burden among patients, and can be used to inform RSV immunization implementation decisions. METHODS: We estimated the hospital burden of RSV-associated RTI (RSV-RTI) in children under 5 years in 7 European countries (Finland, Denmark, Norway, Scotland, England, the Netherlands, and Italy) using routinely collected hospital databases during 2001-2018. We described RSV-RTI admission rates during the first year of life by birth month and assessed their correlation with RSV seasonality in 5 of the countries (except for England and Italy). We estimated average annual numbers and rates of bed days for RSV-RTI and other-pathogen RTI, as well as the hospital LOS. RESULTS: We found that infants born 2 months before the peak month of RSV epidemics more frequently had the highest RSV-RTI hospital admission rate. RSV-RTI hospital episodes accounted for 9.9-21.2 bed days per 1000 children aged <5 years annually, with the median (interquartile range) LOS ranging from 2 days (0.5-4 days) to 4 days (2-6 days) between countries. Between 70% and 89% of these bed days were in infants aged <1 year, representing 40.3 (95% confidence interval [CI], 40.1-40.4) to 91.2 (95% CI, 90.6-91.8) bed days per 1000 infants annually. The number of bed days for RSV-RTI was higher than that for RTIs associated with other pathogens in infants aged <1 year, especially in those <6 months. CONCLUSIONS: RSV disease prevention therapies (monoclonal antibodies and maternal vaccines) for infants could help prevent a substantial number of bed days due to RSV-RTI. "High-risk" birth months should be considered when developing RSV immunization schedules. Variation in LOS between countries might reflect differences in hospital care practices.

Reduced risk of hospitalisation among reported COVID-19 cases infected with the SARS-CoV-2 Omicron BA.1 variant compared with the Delta variant, Norway, December 2021 to January 2022.

We included 39,524 COVID-19 Omicron and 51,481 Delta cases reported in Norway from December 2021 to January 2022. We estimated a 73% reduced risk of hospitalisation (adjusted hazard ratio: 0.27; 95% confidence interval: 0.20-0.36) for Omicron compared with Delta. Compared with unvaccinated groups, Omicron cases who had completed primary two-dose vaccination 7-179 days before diagnosis had a lower reduced risk than Delta (66% vs 93%). People vaccinated with three doses had a similar risk reduction (86% vs 88%).

The burden of respiratory syncytial virus in children under 5 years of age in Norway.

OBJECTIVES: To estimate age-specific incidence of medically attended respiratory syncytial virus (RSV) infections in hospitalised Norwegian children and describe disease epidemiology. METHODS: Active prospective hospital surveillance for RSV in children <59 months of age was conducted during 2015-2018. All febrile children 12-59 months of age were enrolled, whereas children <12 months were enrolled based on respiratory symptoms regardless of fever. Surveillance data were linked to national registry data to estimate the clinical burden of RSV. RESULTS: Of the children enrolled, 1096 (40%) were infected with RSV. The highest incidence rates were found in children 1 month of age, with a peak incidence of 43 per 1000 during the 2016-2017 season. In comparison, children 24-59 months of age had an infection rate of 1.4 per 1000 during the same winter season. The peak season was during the 2016-2017 winter, with an incidence rate of 6.0 per 1000 children 0-59 months of age. In the study population a total of 168 (15%) of the infected children had pre-existing medical conditions predisposing for more severe disease. High infection rates were found in this population. CONCLUSIONS: Children with comorbidities showed high hospital contact rates, but the majority of children in need of medical attention associated with RSV infection were previously healthy.