Fully vaccinated people can contract and pass on COVID-19 in the home but at lower rates than unvaccinated people. These are the findings of a study of COVID-19 transmission between household contacts, led by Imperial College London and the UK Health Security Agency (HSA) and reported in a news release.
The study was published in The Lancet Infectious Diseases.
It finds that people who have received two doses of vaccine have a lower, but still appreciable, risk of becoming infected with the Delta variant in the home compared with people who are unvaccinated. The authors stress that vaccination also reduces the risk of severe illness, hospitalization and death from COVID-19.
The analysis found that around 25% of vaccinated household contacts tested positive for COVID-19 compared with roughly 38% of unvaccinated household contacts.
Fully vaccinated people cleared the infection more quickly than those who are unvaccinated, but their peak viral load — the greatest amount of SARS-CoV-2 virus found in their nose and throat — was similar to that seen in unvaccinated people, which may explain why they can still readily pass on the virus in household settings.
Despite transmission between vaccinated people being possible, the researchers say it is essential for people who are unvaccinated, and those who are now eligible for boosters, to get vaccinated against COVID-19 to protect themselves from severe disease and hospitalization.
In the study, carried out by the NIHR Health Protection Research Unit in Respiratory Infections at Imperial College London, researchers enrolled 621 participants, identified by the UK contact tracing system, between September 2020 and September 2021, which was before vaccine boosters had become widely available in the UK.
All participants had mild COVID-19 illness or were asymptomatic (showing no symptoms) and took swabs from their nose and throat each day for 14-20 days.
Of the 621 participants, 163 tested positive for COVID-19. Whole genome sequencing confirmed that 71 were infected with the Delta variant of the SARS-CoV-2 virus, 42 had Alpha and 50 had the original strain of SARS-CoV-2. Of the 71 participants infected with Delta, 23 (32%) were unvaccinated, 10 (14%) received one vaccine dose and 38 (54%) had received two vaccine doses.
A total of 205 household contacts of Delta variant index cases were identified, of which 53 tested positive for COVID-19. Of the 205 contacts, 126 (62%) had received two vaccine doses, 39 (19%) had received one vaccine dose, and 40 (19%) were unvaccinated.
Among household contacts who had received two vaccine doses, 25% (31/126 contacts) became infected with the Delta variant compared with 38% (15/40) of unvaccinated household contacts.
Among vaccinated contacts infected with the Delta variant, the median length of time since vaccination was 101 days, compared with 64 days for uninfected contacts. This suggests that the risk of infection increased within 3 months of receiving a second vaccine dose, due to waning protective immunity. The
A total of 133 participants had their daily viral load trajectories analyzed, of whom 49 had pre-alpha and were unvaccinated, 39 had alpha and were unvaccinated, 29 had delta and were fully vaccinated, and 16 had delta and were unvaccinated.
The analysis found that viral load declined most rapidly among vaccinated people infected with the delta variant compared with unvaccinated people with delta, alpha, or pre-alpha.
However, the peak levels of virus in vaccinated people were similar to those in unvaccinated people. The researchers believe this may explain why the delta variant is still able to spread despite vaccination.
