While COVID-19 vaccination is known to reduce both infection and disease severity, uncertainty remains over whether infected, vaccinated individuals can still transmit the virus.
The UK COVID-19 vaccination programme began in England in December 2020 and while the initial target group were the elderly, it has now been extended to younger individuals. Those who have tested positive for the virus are asked to self-isolate although this is not always possible within a large family household due to the use of shared facilities such as bathrooms. Moreover, it has become apparent that household transmission remains an important route for infection with one recent analysis estimating a transmission rate of 16.6%. Thus measuring the incidence of subsequent household infections could serve as a proxy for the effectiveness of vaccination at reducing the transmission of the virus. This was the subject of a study conducted by a team from Public Health England who set out to determine whether individuals who have received a single dose of vaccine but still became infected, were less likely than unvaccinated individuals, to transmit to other unvaccinated household members. The team used the HOSTED dataset and which contains sociodemographic data for cases and household contacts as well as the date and type of vaccine administered. The dataset is linked with national surveillance system containing information on laboratory confirmed positive COVID-19 test results and also linked to individuals who share the same address, using their National Health Service number. The team defined an index case as a vaccinated individual who became infected and a secondary case as a known household contact with a positive test. A comparison was also made between the two COVID-19 vaccines, BNT162b2 and ChAdOx1.
Data collection occurred between early January and the end of February 2021 and secondary cases up to the middle of March 2021. The final cohort consisted of 365,447 households with a single index case and 1,018,843 contacts. In households where the index case was unvaccinated before testing positive, there were 96,898 secondary cases compared to 196 secondary cases where the index case had been vaccinated 21 days or more before testing positive and 371 secondary cases. This resulted in an adjusted odds ratio for being a secondary case if the index case was vaccinated (vs unvaccinated) with the ChAdOx1 of 0.53 (95% CI 0.43 – 0.63) and for BNT162b2, 0.51 (95% CI 0.44 – 0.59).
In commenting on these findings, the authors highlighted that their data indicate that among those who have been vaccinated 21 or more days before becoming infected with COVID-19, household transmission is 40 – 50% lower compared to those who are unvaccinated. Moreover, there are appears to be little differences between the two vaccines. The authors concluded that while the data specifically relates to household contacts, it is possible that a reduction in transmissibility is also likely in other settings.
Harris RJ et al. Impact of vaccination on household transmission of SARS-COV-2 in England. Public Health England 2021