Vaccinations that can block viral transmissions will help control the pandemic.Credit: Andrea Fasani / EPA-EFE / Shutterstock
As countries roll out vaccines that prevent COVID-19, studies are underway to determine whether shots can also prevent people from becoming infected and transmitting the SARS-CoV-2 virus. Vaccinations that prevent transmission can help bring the pandemic under control if given to enough people.
Preliminary analyzes suggest that at least some vaccines are likely to have a transmission-blocking effect. But confirming the effect – and how strong it will be – is tricky, because a decrease in infections in a particular region can be explained by other factors, such as closures and behavioral changes. Not only that, the virus can spread from asymptomatic carriers, making it difficult to detect those infections.
“This is one of the most difficult types of studies to do,” says Marc Lipsitch, an epidemiologist at Infectious Diseases at the Harvard TH Chan School of Public Health in Boston, Massachusetts. “All of us are out there trying hungry to see what we can get out of small pieces of data that do come out,” he says. Results of some studies are expected in the next few weeks.
Stop infections?
Although most clinical trials with COVID-19 vaccines have shown that vaccines prevent the disease, some results have also given clues that shot can prevent infection. A vaccine that is primarily effective in preventing people from contracting the infection can reduce transmission, says Larry Corey, a vaccine specialist at the Fred Hutchinson Cancer Research Center in Seattle, Washington.
During the trial of the Moderna vaccine, which was manufactured in Boston, researchers screened all participants to see if they had any viral RNA. They saw a two-thirds decrease in the number of asymptomatic infections among people who received the first shot of the two-dose vaccine compared to those who received a placebo. But they only tested humans twice, about a month apart, so they might have missed infections.
The UK trial of the vaccine produced by the University of Oxford and AstraZeneca tested participants weekly and estimated a 49.3% decrease in asymptomatic infections among a subgroup of vaccinated participants compared to the non-vaccinated group. not.
Pfizer, based in New York City and manufacturer of another leading COVID-19 vaccine, says it will begin vaccinating participants in vaccination trials taking place in the United States and Argentina every two weeks to see if the shot infection can occur.
Less contagious?
It is possible that vaccines will not stop or significantly reduce the chance of infection. But jabs can make infected people less able to transmit the virus, or make it less contagious, thus reducing transmission.
Several research groups in Israel measure ‘viral load’ – the concentration of viral particles in vaccines that are later positive for SARS-CoV-2. Researchers have found that viral load is a good proxy for infectivity1.
In preliminary work, one team observed a significant decrease in viral load in a small number of people infected with SARS-CoV-2 during the two to four weeks after receiving their first dose of Pfizer vaccine, compared to those who have the virus in the first two weeks after the injection2. “The data are certainly intriguing and suggest that vaccination may reduce the contagion of COVID-19 cases, even if it can not completely prevent infection,” said Virginia Pitzer, a model for infectious diseases at the Yale School of Public Health New Haven, Connecticut. . The Oxford-AstraZeneca trial also observed a greater decrease in viral load in a small group of vaccinated participants than in the non-vaccinated group.
But whether the observed decrease in viral load is sufficient to make someone less contagious in real life is not yet clear, researchers say.
Gold standard
To really determine if vaccines prevent the transmission, researchers are monitoring the close contact of vaccines to see if they are indirectly protected against infection.
As part of an ongoing study among hundreds of health workers in England, known as PANTHER, researchers at the University of Nottingham tested health workers and the people they lived with for SARS-CoV-2 antibodies and viral RNA between April and August last year. , around the time of the first pandemic wave. They will now test some of the workers after receiving the Pfizer vaccine, as well as their close contact that would not be vaccinated, to see if the risk of infection for the close contacts has decreased, says Ana Valdes, a geneticist epidemiologist. at the University of Nottingham. If the risk decreases, it could mean the vaccines are likely to prevent transmission, Valdes says.
Other groups, in Israel, also plan to study households in which one member has been vaccinated. If these people become infected, researchers can see if they are transmitting the virus to other households.
In Brazil, a dissertation will randomly distribute doses of the COVID-19 vaccine manufactured by Beijing-based pharmaceutical company Sinovac to the city of Serrana over several months. This approach may indicate whether decreases in COVID-19 in vaccinated regions also contribute to reduced transmission in unvaccinated areas. This will show the indirect effects of vaccines, says Nicole Basta, an infectious epidemiologist at McGill University in Montreal, Canada.
Studies of individuals and larger populations are needed to see how well vaccines protect against transmission, Basta says. “We really need evidence that spans the entire spectrum.”