A recent study in the Netherlands, led by scientists from the Center for Infectious Disease Control and the National Institute for Public Health and the Environment (RIVM), sought to estimate vaccine effectiveness against onward viral transmission by comparing secondary attack rates among household members between vaccinated and unvaccinated index cases. Capitalizing on contact tracing data as well as direct source material, the investigators conducted the study during the height of the Delta variant in Holland. The study team published yet-to-be peer-reviewed data suggesting that vaccine effectiveness in preventing viral transmission (from fully vaccinated to fully vaccinated) equals 40% (95% CI 20-54%). The effectiveness of full vaccination against transmission to unvaccinated household contacts equaled 63% (95% CI 46-75%). The study team reminded the reader that a previous study indicated that regarding the Alpha variant the effectiveness was 73% (95% CI 65-79%) against transmission to unvaccinated household contacts. The study was made possible by data accumulated by personnel at 25 Municipal Health Services agencies.

This Dutch observational population study, led by a team known as “RIVM COVID-19 surveillance and epidemiology team,”  amounts to an estimation and includes limitations. Led by Brechije de Gier, the Netherlands-based study team used data from the national contact tracing system to estimate the transmissibility of SARS-CoV-2 in the Netherlands. Leveraging contact tracing system data, they sought to better understand the “secondary attack rate,” that is the percentage of SARS-CoV-2-infected cases lowered by vaccination.

The study investigators wrote in medRxiv that the results of their population-based study indicate that those who are COVID-19 vaccinated have protection against transmission from “vaccinated index cases.” Finding that these results are stronger in Alpha than the more virulent and transmissible Delta, they conclude “Vaccine effectiveness against transmission to unvaccinated household contacts is stronger than to vaccinated household contacts.” They emphasize that the vaccinated enjoy greater protection subject to variation based on factors such as individual behavior.

Importantly, the study results acknowledge the real-world observations involving vaccine durability challenges. The waning of vaccine effectiveness against infection and transmission of others, of course, can result in an increase in SARS-CoV-2 circulation in various populations. Based on the premise that full vaccination remains “highly effective in preventing severe disease” the authors argue vaccination remains front and center as input for the war on COVID-19.

Relevant Data

The data depicted in the table above was downloaded from the supplementary materials associated with the study. Based on data collected by personnel at 25 Municipal Health Services agencies during the late summer, the Delta variant was the dominant strain of the virus.

Note, the drop in SAR from 22% to 13% is associated with vaccinated index cases. Noteworthy, under “fully vaccinated households” SAR increases from 11% to 12%. The authors suggest that vaccinated index cases associated with reduced SAR demonstrate less transmissibility. However, questions have arisen. Note, greater transmissibility is associated with the 50-74 cohort.

Study Funding

Dutch Ministry of Health, Welfare, and Sports.

Lead Research/Investigator 

Brechije de Gier, Center for Infectious Disease Control, National Institute for Public Health and Environment (RIVM)

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