As countries around the world roll out vaccines, studies are still ongoing to determine whether COVID-19 vaccines can also stop people from getting infected and passing on the SARS-CoV-2 virus to others. Vaccines that prevent transmission could accelerate the end of the pandemic if they are given to enough people.
Some vaccines may block the transmission of the virus, preliminary analyses suggest. However, it is not easy to confirm this effect because a drop in infections in a given region could be due to other factors, such as lockdowns.
Another big challenge is that the SARS-CoV-2 can spread from asymptomatic carriers. This makes it hard to detect such infections.
Researchers are of the view that these are some of the hardest types of studies to do.
Almost all clinical trials of COVID-19 vaccines showed that vaccines prevented the disease. Data also indicate that some vaccines might make infected people less able to pass the virus on to others. To demonstrate this, researchers are measuring viral load in an individual as a good indicator of infectiousness.
Recently, one team of researchers found a significant drop in viral load in a small number of people infected with SARS-CoV-2 in the first two to four weeks after receiving their first dose of the Pfizer vaccine, compared with those who caught the virus in the first two weeks after the injection. It suggests that vaccination may reduce the infectiousness of COVID-19 cases, even if it does not prevent infection altogether.
But these observations need to be analysed in real-life situations to make it clear, say researchers.
Smaller studies are underway in the UK, Israel and Brazil to determine how well vaccines protect against transmission. But researchers say larger studies are required to establish this link.