Can COVID vaccines stop transmission? Scientists rush to find answers

As countries roll out vaccines that prevent COVID-19, studies are underway to determine whether vaccines can also prevent people from becoming infected and transmitting the SARS-CoV-2 virus. Vaccines that prevent transmission could help control the pandemic if given to enough people.

Preliminary analyzes suggest that at least some vaccines are likely to have a transmission blocking effect. But confirming this effect – and its strength – is tricky because a drop in infections in a given region could be explained by other factors, such as lockouts and changes in behavior. In addition, the virus can spread from asymptomatic carriers, making it difficult to detect these infections.

“These are among the most difficult types of studies to perform,” says Marc Lipsitch, infectious disease epidemiologist at Harvard TH Chan School of Public Health in Boston, Massachusetts. “We’re all out there, eagerly trying to see what we can squeeze out of the little bits of data that comes out,” he says. The results of some studies are expected in the coming weeks.

Stop infections?

While most clinical trials of COVID-19 vaccines have shown that vaccines prevent disease, some trial results have also offered clues that injections may prevent infection. A vaccine that is very effective at preventing people from getting the infection in the first place would help reduce transmission, says Larry Corey, a vaccinologist at the Fred Hutchinson Cancer Research Center in Seattle, Washington.

During the trial of Moderna’s vaccine, produced in Boston, the researchers took samples from all the participants to see if they had viral RNA. They saw a two-thirds drop in the number of asymptomatic infections in people who received the first two-dose vaccine, compared to those who received a placebo. But they only tested people twice, about a month apart, so they might have missed out on infections.

The UK trial of the vaccine produced by the University of Oxford and AstraZeneca sampled participants weekly and estimated a 49.3% reduction in asymptomatic infections among a subset of vaccinated participants compared to the group not vaccinated.

New York-based Pfizer and maker of another leading COVID-19 vaccine, says it will begin rubbing participants every two weeks in vaccine trials taking place in the United States and Argentina, to see whether the vaccine can prevent infection.

Less contagious?

Vaccines may not stop or greatly reduce the risk of infection. But the bites can make infected people less able to transmit the virus, or make them less infectious, and thus reduce transmission.

Several research groups in Israel are measuring “viral load” – the concentration of viral particles in vaccinated people who will later test positive for SARS-CoV-2. Researchers have found that viral load is a good indicator of infectivity¹.

In preliminary work, a team observed a significant drop in viral load in a small number of people infected with SARS-CoV-2 within two to four weeks of receiving their first dose of Pfizer vaccine, compared to those who caught the virus within the first two weeks after the injection². “The data is certainly intriguing and suggests that vaccination may reduce the infectivity of COVID-19 cases, even if it does not completely prevent infection,” says Virginia Pitzer, infectious disease modeler at the Yale School of Public Health of New Haven, Connecticut. The Oxford-AstraZeneca trial in the UK also observed a greater reduction in viral load in a small group of vaccinated participants than in the unvaccinated group.

But whether these observed reductions in viral load are enough to make a person less contagious in real life, it’s not yet clear, the researchers say.

Gold standard

To really determine whether vaccines prevent transmission, researchers follow close contact with vaccinated people to see if they are indirectly protected from infection.

In an ongoing study of hundreds of healthcare workers in England known as PANTHER, researchers at the University of Nottingham tested healthcare workers and the people they lived with for them. anti-SARS-CoV-2 antibody and viral RNA between April and August, at the time of the first pandemic wave. They will now retest some of these workers after receiving the Pfizer vaccine, as well as their close contacts who will not have been vaccinated, to see if the risk of infection has decreased for close contacts, says Ana Valdes, a genetic epidemiologist. at the University of Nottingham. If the risk decreases, that would mean the vaccines are likely to prevent transmission, Valdes says.

Other groups in Israel are also planning to study households in which a member has been vaccinated. If these people are infected, researchers can see if they are transmitting the virus to other members of the household.

In Brazil, a trial will randomly distribute doses of the COVID-19 vaccine produced by the Beijing-based pharmaceutical company Sinovac in the city of Serrana in stages over several months. This approach could show whether declines in COVID-19 in vaccinated areas are also helping to reduce transmission in unvaccinated areas. This would demonstrate the indirect effects of vaccines, says Nicole Basta, infectious disease epidemiologist at McGill University in Montreal, Canada.

Studies on larger individuals and populations are needed to see how well vaccines protect against transmission, Basta says. “We really need evidence that covers the whole spectrum.”