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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.
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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.
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BackgroundWe aimed to estimate vaccine effectiveness (VE) against COVID-19 mortality, and to explore whether an increased risk in non-COVID-19 mortality exists in the weeks following a COVID- 19 vaccine dose. MethodsNational registries of causes of death, COVID-19 vaccination and long-term care reimbursements were linked by a unique identifier using data from 1 January 2021 to 31 January 2022. We used Cox regression with calendar time as underlying time scale to, firstly, estimate VE against COVID-19 mortality after primary and first booster vaccination, per month since vaccination and, secondly, estimate risk of non-COVID-19 mortality in the 8 weeks following a first, second or booster dose, adjusting for birth year, sex and country of origin. ResultsVE against COVID-19 mortality was >90% for all age groups two months after completion of the primary series. VE gradually decreased thereafter, to around 80% at 7-8 months post-primary series for most groups, and around 60% for elderly receiving a high level of long-term care and for people aged 90+ years. The risk of non-COVID-19 mortality was lower or similar in the 8 weeks following a first booster dose compared to no vaccination, first or second dose, respectively, for all age and long-term care groups. ConclusionCOVID-19 vaccination greatly reduced the risk of COVID-19 mortality and no increased risk of death from other causes was seen at the population level.
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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.
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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.
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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.
