By Mariana Bego
Researchers from Public Health England followed COVID-19 vaccine recipients nationwide to measure their risk of infection and transmission. In a preprint, therefore not yet peer reviewed, they suggest that household transmission was cut by 40-50% if the person had received one dose of either the Pfizer-BioNTech or AstraZeneca vaccine before testing positive. Meantime, a study in the U.S. suggests a significant gain in protection with two doses of the Pfizer-BioNTech or Moderna vaccines.
A single dose of COVID-19 vaccine offers protection to your unvaccinated household contacts
- If the COVID-positive person was not vaccinated beforehand, 10% of the possible contacts became infected.
- When the COVID-positive person was previously vaccinated, transmission to unvaccinated household contacts was reduced to almost half.
- Transmission was 5.72% when the COVID-positive person had had the AstraZeneca vaccine.
- Transmission was 6.25% when the COVID-positive person had had the Pfizer-BioNTech vaccine.
Many studies have already demonstrated that vaccination against COVID-19 significantly reduces your risk of being infected, but evidence shows that some can still get infected after receiving a single vaccine dose. Household contacts of confirmed cases have the highest risk of infection, as this setting is a key route of transmission. Due to this inherent risk, households can provide early evidence for the impact of vaccines on transmission. Researchers from Public Health England sought to determine the likelihood that someone who got SARS-CoV-2 after having a single dose could infect their unvaccinated household contacts.
The study group: For this study, over 350,000 residential households of 2 to 10 people were included. Participating households had at least one person with a confirmed COVID-19 test and the pooled data added up to approximately 1 million contacts. In 4,107 of these households, the person was vaccinated 21 days or more before testing positive (1.12%), and in 20,110 the cases, the person was vaccinated less than 21 days before testing positive (5.51%).
The protection numbers:
- If the COVID-positive person was not vaccinated beforehand, 10% of the possible contacts became infected (96,898 secondary cases in 960,765 contacts).
- When the COVID-positive person was previously vaccinated, transmission to unvaccinated household contacts was reduced to almost half.
- Transmission was 5.72% when the COVID-positive person had had the AstraZeneca vaccine (196 secondary cases in 3,424 contacts).
- Transmission was 6.25% when the COVID-positive person had had the Pfizer-BioNTech vaccine (371 secondary cases in 5,939 contacts).
The study had two major limitations. The authors noted that their data could not identify asymptomatic cases. It is also possible that many of the ‘secondary’ cases included could actually be co-primary, as the two individuals could have been infected by a common third party.
Finally, the authors concluded that the likelihood of household transmission is 40-50% lower for households in which the COVID-positive person was vaccinated 21 days or more prior to testing positive (compared to no vaccination), with comparable effects for both AstraZeneca and Pfizer-BioNTech vaccines. They highlighted that over 90% of those who tested positive after being vaccinated had only received one dose and that it will be important to assess if there is any further reduction in transmissibility after the second dose of vaccine is administered.
While the results analyzing the benefits from a single dose are quite encouraging, a recent study by the US Department of Health and Human Services/Centers for Disease Control and Prevention highlights the advantages of being fully vaccinated against SARS-CoV-2. Their study of a multistate network of U.S. hospitals between January and March 2021, vaccination with either Pfizer-BioNTech or Moderna COVID-19 vaccines was 94% effective against COVID-19 hospitalization among fully vaccinated adults but only 64% effective among partially vaccinated adults aged ≥65 years.
Harris RJ, Hall JA, Zaidi A, Andrews NA, Dunbar JK, Dabrera G. Impact of vaccination on household transmission of SARS-COV-2 in England. Pre-print K Hub Net. https://khub.net/documents/135939561/390853656/Impact+of+vaccination+on+household+transmission+of+SARS-COV-2+in+England.pdf/35bf4bb1-6ade-d3eb-a39e-9c9b25a8122a?t=1619601878136