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, the researchers said.This suggests that the risk of infection increased within three months of receiving a second vaccine dose, likely due to waning protective immunity, they said.The authors point to vaccine waning as important evidence for all eligible people to receive booster shots.A total of 133 participants had their daily viral load trajectories analysed, of whom 49 had pre-Alpha variant and were unvaccinated, 39 had Alpha and were unvaccinated, 29 had Delta and were fully vaccinated, and 16 had Delta and were unvaccinated.The study found that the viral load declined more rapidly among vaccinated people infected with the Delta variant compared with unvaccinated people with Delta, Alpha, or pre-Alpha.However, the authors note that vaccinated people did not record a lower peak viral load than unvaccinated people, which may explain why the Delta variant can still spread despite vaccination as people are most infectious during the peak viral load phase.”Understanding the extent to which vaccinated people can pass on the Delta variant to others is a public health priority,” said Anika Singanayagam, co-lead author of the study.”By carrying out repeated and frequent sampling from contacts of COVID-19 cases, we found that vaccinated people can contract and pass on infection within households, including to vaccinated household members,” Singanayagam said.The findings suggest that continued public health and social measures to curb transmission such as mask wearing, social distancing, and testing remain important, even in vaccinated individuals.The authors acknowledge some limitations to their study. Due to the nature of UK symptoms-based community testing, only contacts of symptomatic index cases were recruited, the researchers said.As the study was undertaken when infection was circulating widely, it cannot be excluded that another household member may already have been infected and transmitted COVID-19 to the index case, they added.
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