The study found that of 205 household contacts of people who had the Delta infection, 38% of household contacts who were unvaccinated tested positive for COVID-19, and 25% of vaccinated contacts tested positive for the Delta variant of coronavirus.
"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," Dr. Anika Singanayagam, co-lead author of the study, said.
Ajit Lalvani, a professor of infectious diseases at Imperial College London who co-led the study, said, "Vaccines are critical to controlling the pandemic, as we know they are very effective at preventing serious illness and death from COVID-19. However, 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," Lalvani added.
The study revealed that the median length of time since vaccination that the vaccinated infected contacts were recruited for the study was 101 days compared to an average of 64 days for uninfected contacts. This could point to a timeframe when COVID-19 vaccines lose their protective immunity — which is a much shorter time period than previously reported.
Lalvani, who is the director of the Health Protection Research Unit in Respiratory Infections for the National Institute for Health Research, was shocked by how quickly the vaccine lost its efficacy and advised people to get booster shots.
"What we found, surprisingly, was that by three months after the second vaccine dose, the risk of acquiring infection was high compared with being more recently vaccinated," Lalvani stated. "This suggests that vaccine induced protection is already waning by about three months after the second dose."
"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," he recommended.
Imperial epidemiologist Neil Ferguson said, "Immunity wanes over time, it is imperfect, so you still get transmission happening, and that is why the booster program is so important."
Singanayagam hypothesized, "Our findings provide important insights into... why the Delta variant is continuing to cause high COVID-19 case numbers around the world, even in countries with high vaccination rates."
In April, Pfizer and BioNTech proclaimed that their COVID-19 vaccine provides protection for at least six months.
The Centers for Disease Control and Prevention recommends that eligible people get the booster at least six months after their second shot. The CDC says those who are eligible to receive a booster shot include anyone age 65 and older and anyone over 18 who is in long-term care settings, has an underlying medical condition, or works/lives in high-risk settings.