The deployed coronavirus disease 2019 (COVID-19) vaccines have been shown to provide robust immune responses for those administered. However, little investigation has taken place into what effect vaccination may have on the transmissibility of the virus via these people.
In a new study, recently released on the medRxiv* preprint server, the ability of COVID-19 vaccines to lessen transmission was examined in a large study of healthcare workers and their families, with promising results that indicate a substantially reduced rate of transmission from the vaccinated to the non-vaccinated.
How was the study performed?
A number of previous studies have suggested that lessened SARS-CoV-2 transmission following vaccination is probable. However, these studies either did not account for occupation or were too small to include hospitalization rates. Occupation is a particularly important factor with regard to healthcare workers as they are both more likely to receive a vaccine, and are in the high-risk group due to increased exposure. Therefore, examining the non-vaccinated family members of healthcare workers was determined to be a good indicator of transmissibility.
Working-age (18-65) healthcare workers in the UK that had not before had a positive SARS-CoV-2 test and shared a home with non-healthcare workers were included in the study, totaling almost 150,000 healthcare workers and 200,000 household members. The vaccination program was initiated for the NHS workers on December 8, 2020, and outcomes including positive COVID-19 PCR test, hospitalization with COVID-19, and death from COVID-19 were accounted for all participants up until March 3, 2021. 78.3% of the healthcare workers had received at least one dose of the vaccine, while 25.1% had also received the second at the time of the study.
Did the vaccines lessen transmission?
Overall, among the non-vaccinated household members of vaccinated healthcare workers, there was an observed 30% reduction in documented cases 14 days post-vaccination, and a non-significant reduction in hospitalizations. Even within 1-7 days following the first dose of the vaccine, both healthcare workers and their non-vaccinated household members exhibited fewer new SARS-CoV-2 infections. However, the group suggests this may be the product of bias introduced by the already-ill avoiding vaccination centers.
As the non-vaccinated household members of healthcare workers can also obtain infections from other sources, besides the healthcare worker with which they coinhabit, the 30% reduction in transmission could, in fact, be a severe underestimation. The group attribute around half of these cases to sources other than the coinhabiting healthcare worker, based on previous estimates, and thus the true drop in transmission is estimated to be around 60% having received a single dose of the vaccine. Vaccination lessens the asymptomatic carriage of SARS-CoV-2, as has been demonstrated in some phase III clinical studies, which provides a reasonable explanation behind the observed drop in transmission.
Arguments regarding the administration of a single, two, or a delayed second dose of the vaccine have been put forth for each case, with wider but less complete immunity throughout the population perhaps being optimal. At the time of this study, 25,000 workers had received the second dose of the vaccine, allowing the group to assess the influence of a second dose on transmissibility. Household members of workers having received two doses exhibited a similarly large drop in virus acquisition as observed in household members of those receiving one dose compared to none, around 30%.
Hospitalization rates were low but consistent across those that had or had not been vaccinated, suggesting that while transmissibility lessened. The resulting severity of disease, however, was not altered. This validates and supports the reported drop in transmissibility, as bias introduced by differences in behavior and other factors are clearly evened out amongst the population. Importantly, this study supports the vaccination of frontline healthcare workers as effective protection for the wider community.
medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.
- Anoop S V Shah, Ciara Gribben, Jennifer Bishop, Peter Hanlon, David Caldwell, Rachael Wood, Martin Reid, Jim McMenamin, David Goldberg, Diane Stockton, Sharon Hutchinson, Chris Robertson, Paul M McKeigue, Helen M Colhoun, David A McAllister. Effect of vaccination on transmission of COVID-19: an observational study in healthcare workers and their households. medRxiv preprint server. doi: https://doi.org/10.1101/2021.03.11.21253275, https://www.medrxiv.org/content/10.1101/2021.03.11.21253275v1