Vaccinated Individuals Can Still Contract and Transmit Delta in Household Settings

Most COVID-19 infection occurs in household settings, and though vaccinated individuals who contracted the Delta variant recovered more quickly than unvaccinated persons, they had similar peak viral load.

Vaccines remain highly effective at preventing severe disease, hospitalization, and death, but vaccinated individuals can still contract and spread COVID-19.

In research released today by The Lancet, one study reported that people with 2 doses of a COVID-19 vaccine have a lowered, but not eradicated, risk of contracting the Delta variant.

Most COVID-19 spreading occurs in households, so the study sought participants who were household contacts with possibly exposure to the Delta variant. The study enrolled 621 participants from September 2020-September 2021. Participants were identified by the UK contact tracing system, and all had asymptomatic to mild COVID-19 disease. Participants completed daily PCR tests for 14-20 days to detect infection and viral load.

Investigators identified 205 household contacts of Delta variant cases, 53 of whom PCR-tested positive. Of the 205 contacts, 125 (62%) received 2 vaccine doses, 39 (19%) had one vaccine dose, and 40 (19%) were unvaccinated. Among the fully vaccinated household contacts, 25% (31/126) became infected with Delta, as compared to a 38% (15/40) infection rate among unvaccinated household contacts. Investigators analyzed the daily viral load of 133 participants; 49 had a pre-Alpha strain and were unvaccinated, 39 had Alpha and were unvaccinated, 29 had Delta and were fully vaccinated, and 16 had Delta and were unvaccinated.

Vaccine efficacy wanes over time, and this was reflected in the data. Among vaccinated persons who contracted Delta, the median length of time since vaccination was 101 days, versus 64 days for uninfected household contacts.

Fully vaccinated persons who do contract Delta recover more quickly than unvaccinated individuals, but they have similar peak viral loads. Viral load declined more rapidly in vaccinated people infected with Delta (0.95 log10 virus copies/mL/day) in comparison to unvaccinated people with Delta (0.79), Alpha (0.82), or pre-Alpha (0.69). Notably, 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.

According to Professor Ajit Lalvani of Imperial College London, UK, co-leader of the study, “Our findings show that vaccination alone is not enough to prevent people from being infected with the delta variant and spreading it in household settings. The ongoing transmission we are seeing between vaccinated people makes it essential for unvaccinated people to get vaccinated to protect themselves from acquiring infection and severe COVID-19, especially as more people will be spending time inside in close proximity during the winter months. We found that susceptibility to infection increased already within a few months after the second vaccine dose—so those eligible for COVID-19 booster shots should get them promptly.”