Study Finds Vaccination Protects Against Further Transmission of SARS-CoV-2


In early August 2021, a research group reported the effectiveness of vaccination against SARS-CoV-2 transmission in the Netherlands by monitoring individuals in close contact with those known to be infected, finding that around 71% of protection against transmission was afforded by vaccination.

Study: Vaccine efficacy against transmission of SARS-CoV-2 to household contacts during Delta variant dominance (B.1.617.2), August-September 2021, The Netherlands. Image Credit: Nicolas Economou / Shutterstock

This study was conducted from February to May 2021, during the rise and peak of dominance of the alpha variant of the coronavirus 2 severe acute respiratory syndrome (SARS-CoV-2). However, in early July of the same year, non-pharmaceutical measures were relaxed in the Netherlands, and the highly transmissible delta variant of SARS-CoV-2 quickly became prominent.

In an update of this work, the research group reports on the decreasing efficacy of 2019 coronavirus disease (COVID-19) vaccines against transmission to household contacts in the face of these relaxed measures and more transmissible strains of SARS-CoV-2.

A pre-printed version of the study is available on the website medRxiv* server while the article is subject to peer review.

How was the study carried out?

The authors say that many cases of the delta variant of SARS-CoV-2 in July left health services unable to fully engage in contact tracing. Thus, incomplete data for this period are excluded from the analysis, as of August 9.e. Likewise, the Netherlands introduced vaccination passport requirements as of September 24.e, which, according to the authors, is likely to distort the behavior of the participants, hence the final date of the study.

Unlike the alpha variant dominance peak, where a variety of SARS-CoV-2 variants were still strongly represented in the population, the delta variant made up over 97% of all COVID-19 cases during this period.

As determined by the source research interviews, index cases who had been infected with SARS-CoV-2 outside the home were eligible for the study. All participants and their family contacts over the age of 12 were then included in the analysis. Each was classified as fully, partially, or unvaccinated, which made it possible to compare the secondary attack rate of SARS-CoV-2 to household contact between groups.

Four thousand nine hundred and twenty-four index cases were included in the study; 53.9% were fully vaccinated and 8.2% were partially vaccinated. The average vaccination rate among adults in the Netherlands is 71%, and therefore the overrepresentation of the unvaccinated in the study group indicates the vaccine’s effectiveness against transmission.

Among the vaccinated index cases, there was an incidence of 11.6% of unvaccinated people living in the same household, while among the unvaccinated, 59.1% of household contacts were also unvaccinated.

Effectiveness of vaccine against transmission

In cases where the index case is vaccinated but the household is not, then the secondary attack rate was 13%, while the unvaccinated index cases induced secondary cases 22% of the time in unvaccinated household contacts. vaccinated, producing an adjusted vaccine efficacy against transmission of 63%.

When household contacts were fully vaccinated, transmission from the index case occurred in 11% and 12% of cases when vaccinated or unvaccinated, respectively. The authors state that the bias related to the fact that older people are more likely to be fully immunized, to live in shared accommodation with other index cases, and to experience a secondary attack likely skewed this result and therefore produced a adjusted efficacy of vaccination against transmission from index cases by 40%.

In the previous study of the alpha variant of SARS-CoV-2, the vaccine’s efficacy against transmission was 71%, possibly due to the observed increased transmissibility of the delta variant. The secondary attack rate among unvaccinated household contacts was lower for the delta variant than for the alpha variant, which the authors argued may be due to a more widespread seroprevalence in the population or in the average younger population. of this study.

A decrease in vaccine efficacy against transmission over time was also noted in this study. People who had received the vaccine more than two months previously were more likely to engage in secondary transmission. Since the most vulnerable population was vaccinated earlier, the observed loss of efficacy against transmission may be due in part to this decreasing protection.


Before July 2021, people positive for COVID-19 and their family contacts had to quarantine themselves at home for ten days. However, the rules have been changed to allow fully vaccinated household contacts to avoid quarantine for now.

In addition, the requirement that household contacts produce a negative COVID-19 test before release from quarantine has been lifted, which likely resulted in many secondary attack infections being ignored by this study and resulting in an overestimation. secondary attack rates. Differences in testing behavior between vaccinated and unvaccinated individuals also skew the results, where asymptomatic unvaccinated individuals are less likely to take and report the results of a test.

In all cases, this study identified a protection rate of 63% against the transmission of SARS-CoV-2 and of 40% against infection. This is lower than the efficacy against transmission observed in previous studies involving the alpha variant of SARS-CoV-2 and is likely the result of the known improved transmissibility of the delta variant in combination with the decrease in vaccine protection in the population.

*Important Notice

medRxiv publishes preliminary scientific reports which are not peer reviewed and, therefore, should not be considered conclusive, guide clinical practice / health-related behavior, or treated as established information.

Journal reference:

  • de Gier, B. et al. (2021) “Vaccine efficacy against SARS-CoV-2 transmission to household contacts during Delta variant dominance (B.1.617.2), August-September 2021, The Netherlands”. medRxiv. do I: 10.1101 / 2021.10.14.21264959.

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