Number of COVID-19 hospitalisations averted by vaccination: Estimates for the Netherlands, August 2, 2021 through August 30, 2022

Background Vaccines against COVID-19 have proven effective in preventing COVID-19 hospitalisation. In this study, we aimed to quantify one aspect of the public health impact of COVID-19 vaccination by estimating the number of averted hospitalisations. We present results from the beginning of the vaccination campaign (period 1, January 6, 2021) and a period starting at August 2, 2021 (period 2) when all adults had the opportunity to complete their primary series, until August 30, 2022. Methods Using calendar-time specific vaccine effectiveness (VE) estimates and vaccine coverage (VC) by round (primary series, first booster and second booster) and the observed number of COVID-19 associated hospitalisations, we estimated the number of averted hospitalisations per age group for the two study periods. From January 25, 2022, when the indication of hospitalisation was registered, hospitalisations not causally related to COVID-19 were excluded. Results In period 1, there were an estimated 98,170 (95% confidence interval (CI) 96,123-99,928) averted hospitalisations, of which 90,753 (95% CI 88,790-92,531) in period 2, equalling 57.0% and 67.9% of all hospital admissions. Estimated averted hospitalisations were lowest for 12-49-year-olds and highest for 70-79-year-olds. More admissions were averted in the Delta period (72.2%) than in the Omicron period (64.0%). Conclusion COVID-19 vaccination prevented a large number of hospitalisations. Although the estimated number of hospitalisations during the study period could not have occurred realistically due to limits on health care, these findings underline the public health importance of the vaccination campaign to policy makers and the public.

Higher risk of SARS-CoV-2 Omicron BA.4/5 infection than of BA.2 infection after previous BA.1 infection, the Netherlands, 2 May to 24 July 2022

We investigate differences in protection from previous infection and/or vaccination against infection with Omicron BA.4/5 or BA.2. We observed a higher percentage of registered previous SARS-CoV-2 infections among 19836 persons infected with Omicron BA.4/5 compared to 7052 persons infected with BA.2 (31.3% vs. 20.0%) between 2 May and 24 July 2022 (adjusted odds ratio (aOR) for testing week, age group and sex: 1.4 (95%CI: 1.3-1.5)). No difference was observed in the distribution of vaccination status between BA.2 and BA.4/5 cases (aOR: 1.1 for primary and booster vaccination). Among reinfections, those newly infected with BA4/5 had a shorter interval between infections and the previous infection was more often caused by BA.1, compared to those newly infected with BA.2 (aOR: 1.9 (1.5-2.6). This suggests immunity induced by BA.1 is less effective against a BA.4/5 infection than against a BA.2 infection.

Pathogen- and type-specific changes in invasive bacterial disease epidemiology during the first year of the COVID-19 pandemic in the Netherlands

The COVID-19 control measures have resulted in a decline in invasive bacterial disease caused by Neisseria meningitidis (IMD), Streptococcus pneumoniae (IPD), and Haemophilus influenzae (Hi-D). The species comprise different serogroups and serotypes that impact transmissibility and virulence. We evaluated type- and pathogen-specific changes in invasive bacterial disease epidemiology in the Netherlands during the first year of the SARS-CoV-2 pandemic. Cases were based on nationwide surveillance for five bacterial species with either respiratory (IMD, IPD, Hi-D) or non-respiratory (controls) transmission routes and compared between the pre-COVID period (April 2015-March 2020) and the first COVID-19 year (April 2020-March 2021). IMD, IPD, and Hi-D cases decreased by 78%, 67%, and 35%, respectively, in the first COVID-19 year compared to the pre-COVID period although effects differed per age group. Serogroup B-IMD declined by 61%, while serogroup W and Y-IMD decreased >90%. IPD caused by serotypes 7F, 15A, 12F, 33F, and 8 showed the most pronounced decline ([≥]76%). In contrast to an overall decrease in Hi-D cases, vaccine-preventable serotype b (Hib) increased by 51%. COVID-19 control measures had pathogen- and type-specific effects related to invasive infections. Continued surveillance is critical to monitor potential rebound effects once restriction measures are lifted and transmission is resumed.

Protection of COVID-19 vaccination and previous infection against Omicron BA.1 and Delta SARS-CoV-2 infections, the Netherlands, 22 November 2021- 19 January 2022

Given the emergence of the SARS-CoV-2 Omicron BA.1 and BA.2 variants and the roll-out of booster COVID-19 vaccination, evidence is needed on protection conferred by primary vaccination, booster vaccination and previous SARS-CoV-2 infection by variant. We employed a test-negative design and used multinomial logistic regression on data from community PCR testing in the Netherlands. S-gene target failure (SGTF) was used as proxy to discern Delta, Omicron BA.1 and Omicron BA.2 infections. Two cohorts were defined to assess protection from vaccination and previous infection by variant: Delta-Omicron BA.1 cohort including data from 22 November 2021 to 7 January 2022 (n = 354,653) and Omicron BA.1-BA.2 cohort including data from 26 January to 31 March 2022 (n = 317,110). In the Delta-Omicron BA.1 cohort, including 39,889 Delta and 13,915 Omicron BA.1 infections, previous infection, primary vaccination or both protected well against Delta infection (76%, 71%, 92%, respectively, at 7+ months after infection or vaccination). Protection against Omicron BA.1 was much lower compared to Delta infections, but BA.1 estimates were imprecise. In the Omicron BA.1-BA.2 cohort, including 67,887 BA.1 and 41,670 BA.2 infections, protection was similar against Omicron BA.1 compared to BA.2 infection for previous infection (34 and 38% at 7+ months post-infection), primary (39 and 32% at 7+ months post-vaccination) and booster vaccination (68 and 63% at 1 month post-vaccination). Higher protection was observed against all variants in individuals with both vaccination and previous infection compared with either one. Protection against all variants by either vaccination or infection decreased over time since last vaccination or infection. Primary vaccination with current COVID-19 vaccines and previous SARS-CoV-2 infections offer low protection against Omicron BA.1 and BA.2 infection. Booster vaccination considerably increases protection against Omicron infection, but decreases rapidly after vaccination.

Increased risk of infection with SARS-CoV-2 Omicron compared to Delta in vaccinated and previously infected individuals, the Netherlands, 22 November to 19 December 2021

Infections by the Omicron SARS-CoV-2 variant are rapidly increasing worldwide. Among 70,983 infected individuals (age [≥] 12 years), we observed an increased risk of S-gene target failure, predictive of the Omicron variant, in fully vaccinated (odds ratio: 5.0; 95% confidence interval: 4.0-6.1) and previously infected individuals (OR: 4.9: 95% CI: 3.1-7.7) compared with infected naive individuals. This suggests a substantial decrease in protection from vaccine- or infection-induced immunity against SARS-CoV-2 infections caused by the Omicron variant compared with the Delta variant.

Increased risk of infection with SARS-CoV-2 Beta, Gamma, and Delta variant compared to Alpha variant in vaccinated individuals

The extent to which severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variants of concern (VOC) break through infection- or vaccine-induced immunity is not well understood. Here, we analyze 28,578 sequenced SARS-CoV-2 samples from individuals with known immune status obtained through national community testing in the Netherlands from March to August 2021. We find evidence for an increased risk of infection by the Beta (B.1.351), Gamma (P.1), or Delta (B.1.617.2) variants compared to the Alpha (B.1.1.7) variant after vaccination. No clear differences were found between vaccines. However, the effect was larger in the first 14-59 days after complete vaccination compared to 60 days and longer. In contrast to vaccine-induced immunity, no increased risk for reinfection with Beta, Gamma or Delta variants relative to Alpha variant was found in individuals with infection-induced immunity.

Vaccine effectiveness against COVID-19 related hospital admission in the Netherlands: a test-negative case-control study

IntroductionReal-world vaccine effectiveness (VE) estimates are essential to identify potential groups at higher risk of break-through infections and to guide policy. We assessed the VE of COVID-19 vaccination against COVID-19 hospitalization, while adjusting and stratifying for patient characteristics. MethodsWe performed a test-negative case-control study in six Dutch hospitals. The study population consisted of adults eligible for COVID-19 vaccination hospitalized between May 1 and June 28 2021 with respiratory symptoms. Cases were defined as patients who tested positive for SARS-CoV-2 by PCR during the first 48 hours of admission or within 14 days prior to hospital admission. Controls were patients tested negative at admission and did not have a positive test during the 2 weeks prior to hospitalization. VE was calculated using multivariable logistic regression, adjusting for calendar week, sex, age, comorbidity and nursing home residency. Subgroup analysis was performed for age, sex and different comorbidities. Secondary endpoints were ICU-admission and mortality. Results379 cases and 255 controls were included of whom 157 (18%) were vaccinated prior to admission. Five cases (1%) and 40 controls (16%) were fully vaccinated (VE: 93%; 95% CI: 81 - 98), and 40 cases (11%) and 70 controls (27%) were partially vaccinated (VE: 70%; 95% CI: 50-82). A strongly protective effect of vaccination was found in all comorbidity subgroups. No ICU-admission or mortality were reported among fully vaccinated cases. Of unvaccinated cases, mortality was 10% and 19% was admitted at the ICU ConclusionCOVID-19 vaccination provides a strong protective effect against COVID-19 related hospital admission, in patients with and without comorbidity.

Vaccine effectiveness against SARS-CoV-2 transmission to household contacts during dominanceof Delta variant (B.1.617.2), August-September 2021, the Netherlands

We estimated vaccine effectiveness against onward transmission by comparing secondary attack rates among household members between vaccinated and unvaccinated index cases, based on source and contact tracing data collected when Delta variant was dominant. Effectiveness of full vaccination of the index against transmission to fully vaccinated household contacts was 40% (95% confidence interval (CI) 20-54%), which is in addition to the direct protection of vaccination of contacts against infection. Effectiveness of full vaccination of the index against transmission to unvaccinated household contacts was 63% (95%CI 46-75%). We previously reported effectiveness of 73% (95%CI 65-79%) against transmission to unvaccinated household contacts for the Alpha variant.

COVID-19 vaccine effectiveness against hospitalizations and ICU admissions in the Netherlands, April- August 2021

The objective of this study was to estimate vaccine effectiveness (VE) against COVID-19 hospitalization and ICU admission, per period according to dominating SARS-CoV-2 variant (Alpha and Delta), per vaccine and per time since vaccination. To this end, data from the national COVID-19 vaccination register was added to the national register of COVID-19 hospitalizations. For the study period 4 April - 29 August 2021, 15,571 hospitalized people with COVID-19 were included in the analysis, of whom 887 (5.7%) were fully vaccinated. Incidence rates of hospitalizations and ICU admissions per age group and vaccination status were calculated, and VE was estimated as 1-incidence rate ratio, adjusted for calendar date and age group in a negative binomial regression model. VE against hospitalization for full vaccination was 94% (95%CI 93-95%) in the Alpha period and 95% (95%CI 94-95%) in the Delta period. The VE for full vaccination against ICU admission was 93% (95%CI 87-96%) in the Alpha period and 97% (95%CI 97-98%) in the Delta period. VE was high in all age groups and did not show waning with time since vaccination up to 20 weeks after full vaccination.

High SARS-CoV-2 attack rates following exposure during singing events in the Netherlands, September-October 2020

BackgroundPrevious reports suggest SARS-CoV-2 transmission risk increases during singing events. From September-October 2020, several clusters of COVID-19 cases among singing events were reported across the Netherlands. Our aim was to investigate whether singing increased SARS-CoV-2 transmission risk during these events. MethodsData from 5 events were retrospectively collected from spokespersons and singing group members via questionnaires. Information was consolidated with the National Notifiable Diseases Surveillance System. Specimens were requested for sequencing for point source and cluster assessment. We described outbreaks in terms of person, place and time and depicted potential SARS-CoV-2 transmission routes. A previously published model (AirCoV2) was used to estimate mean illness risk of 1 person through airborne transmission under various scenarios. ResultsEvents included 9-21 persons (mean: 16), aged 20-89 years (median: 62). Response rates ranged 58-100%. Attack rates were 53-74%. Limited sequencing data was obtained from 2 events. Events lasted 60-150 minutes (singing: 20-120). Rooms ranged 320-3000m3. SARS-CoV-2 transmission likely occurred during all events; with a possible index case identified in 4 events. AirCoV2 showed 86% (54-100%) mean illness risk for 120 minutes of singing, smaller room (300m3), 1 air exchange/hour (ACH), and supershedder presence. ConclusionsDroplet transmission and indirect contact probably caused some cases, but unlikely explain the high attack rates. AirCoV2 indicated that airborne transmission due to singing is possible in case of supershedder presence. Airflow expelling respiratory droplets >1.5m possibly influenced transmission. It is possible that singing itself increased SARS-CoV-2 transmission risk through airborne transmission. SummaryThis outbreak investigation among five singing events with high SARS-CoV-2 attack rates (53-74%) suggested that airflow expelling respiratory droplets >1.5m possibly influenced transmission and it is possible that singing itself increased SARS-CoV-2 transmission risk through airborne transmission.