The fact that this slight decrease was first observed in the oldest age groups is probably because these age groups were also the first to receive the booster vaccination. Waning vaccine effectiveness over time is a possible explanation, but the decrease could also be partly due to the growing number of contacts following the easing of the coronavirus measures. How fast vaccine effectiveness will wane after the COVID-19 booster vaccination will become apparent in the next few weeks and months. Waning protection underlines the importance of a repeat vaccination for certain groups, including people over 70, as the Ministry of Health, Welfare and Sport (VWS) recently decided following an advisory report from the Health Council of the Netherlands.  

Very good protection against Omicron variant immediately after booster vaccination

In the research period from 1 February to 8 March, when 99% of hospital admissions were caused by the Omicron variant, vaccine effectiveness of the basic series in preventing hospital admission was only 48%. A COVID-19 booster vaccination significantly improves protection (vaccine effectiveness 87%). Vaccine effectiveness against ICU admission shows similar improvement: 60% after the basic series, compared to 94% after booster vaccination. This means that the risk of hospital admission after a booster vaccination is 4 times lower than if you have only completed the basic series, and 7 times lower for ICU admission. The low vaccine effectiveness among people who only received the basic series of COVID-19 vaccinations is mainly due to the emergence of the Omicron variant, with waning protection over time after vaccination as a secondary factor. After a booster vaccination or repeat vaccination against COVID-19, vaccine effectiveness increases again in all age groups.

Sources

The analysis is based on NICE data on hospital admissions of patients with COVID-19 admitted until 8 March 2022. The data was enriched on 9 March 2021 with vaccination data from the COVID-19 Information and Monitoring System (CIMS).  The method used to calculate vaccine effectiveness is explained in the report:  Study: Effectiveness of COVID-19 vaccination against hospital and ICU admission in the Netherlands. Since 25 January 2022, new entries in the NICE records include a question regarding the reason for the patient’s admission. Patients who indicated that they were admitted for a reason other than COVID-19 were not included in this analysis.

Please note: the underlying data cannot be published for privacy reasons.