Covid-19: Pfizer checks whether vaccinated people infect others

Even if you’ve had or been vaccinated against Covid-19, for now it is wise to keep wearing a mask to protect others

Sebastián De Toma took part in Pfizer’s clinical study last year and got his shots in August and September. The Argentine journalist still doesn’t know whether he received the real Covid-19 vaccine or the placebo, but on January 31 the study doctors called him with a new offer. Would De Toma be willing to have a series of nasal swabs to be tested for the virus on a regular basis? The doctors had offered to send the Spanish ridesharing service Cabify to take him to the military hospital in Buenos Aires every time. “There they will test me through the car window and that’s it,” says De Toma.

The additional coronavirus tests offered to some volunteers in Argentina and the United States are part of Pfizer’s plan to answer an important Covid question: How often people who are vaccinated develop asymptomatic coronavirus infections despite being vaccinated, and whether they will then continue to spread the virus . This knowledge would be crucial for assessing how the pandemic could develop and how quickly life could return to normal. “The answer will have an impact on how we behave, for example whether we wear masks and whether we trust ourselves to visit restaurants and cinemas,” says Lawrence Corey, operational director of the Covid-19 Prevention Network, which runs several Conducted US vaccine trials.

“There are three things a vaccine can do: keep you from getting the disease, stop it from passing through you, and stop the symptoms,” said Jeffrey Shaman, a public health researcher at Columbia University. A perfect vaccine would create what is known as “sterilizing” immunity, i.e. prevent viruses from even gaining a foothold in the body. Some vaccinations allow low-level infections. The immune system fights this off without symptoms, but a certain amount of virus accumulates in the body, which you may pass on to others.

How well does the broadcast stop?

The question of how well Covid vaccines can stop this transmission is still open because the necessary examinations are expensive and complicated. When companies like Pfizer, Novavax, Moderna Therapeutics, and others launched large studies of their new Covid-19 vaccines last year, they tested whether the vaccines could prevent people with the disease from getting sick or dying. The results were impressive: Hardly anyone who is vaccinated ends up in an intensive care unit with a ventilator.

However, they haven’t looked at whether the vaccines also prevent the virus from spreading – although some computer models have predicted that blocking the transmission could save more lives. A model published by Emory University last August found that a vaccine that does not prevent the disease well but does prevent it from spreading would still prevent more deaths overall because it sufficiently reduces the total number of people infected.

Findings so far suggest that Covid vaccines reduce the likelihood of transmission, but may not eliminate it entirely. For example, vaccinated monkeys that have been sprayed with the virus do become infected , but do not get particularly sick . Overall, they have far fewer viruses in their airways. “There is strong evidence that risk of infection correlates with symptoms. If you can reduce symptoms, you likely reduce transmission,” says Shamane.

Many infected people remain symptom-free

However, that will not be enough, as a large proportion of those infected remain symptom-free. In a report published Jan. 7, epidemiologists from the US Disease Control Agency (CDC) estimated that the proportion of asymptomatic cases is a third and is responsible for about a quarter of all spreads.

While Pfizer is taking an offensive approach, Moderna Therapeutics, the manufacturer of the other RNA vaccine, has not yet answered any questions about whether it is also investigating the transmission rate in vaccinated people. However, preliminary data the company presented to the U.S. FDA in December includes a clue: People who received either vaccine were 66% less likely to test positive for corona than people who received the placebo had. Moderna concludes that some asymptomatic infections are prevented after the first dose.

However, in order to give a conclusive answer to the transmission question, it is not enough just to track down the silent infections. Therefore, researchers of the “Covid-19 Prevention Network” want to test more than 20,000 students at two dozen US locations by “almost daily” nasal swabs in order to monitor exactly when and in what amount the virus in the airways of vaccinated and non-vaccinated students can be found. They then want to use contact tracing to find out how often vaccinated students spread the virus. “We just have to know,” says Corey. “Because we may need to turn our attention to those vaccines that reduce transmission.”

“Pretty well protected grandmother”

This is important for “herd immunity”. The current estimate assumes that it will be reached when around 70 percent of the population is immunized. But when vaccinated people are infectious, the threshold rises. According to basic outbreak math, it is impossible to achieve herd immunity if the vaccine stops less than two-thirds of transmission events. And that doesn’t even take into account that many people might refuse the vaccine or that immunity to new variants of the virus might not last.

According to Jody Lanard, who worked with the World Health Organization as a medical risk communicator, public health officials are likely to send conflicting messages until questions about vaccine delivery are answered. On the one hand, she says, admonishing people to “keep wearing a mask” after vaccination implies that a vaccinated person can continue to transmit the virus – at least until the vaccine has trained the immune system.

At the same time, everyone’s encouragement to get vaccinated “relies heavily on the idea that vaccination is likely to reduce transmission”. She herself still wears a mask permanently after the vaccination, at least until the number of cases in New York, where she lives, drops again. “Thanks to the vaccine, I’m a pretty well protected grandmother now. The last thing I want to do is infect an unprotected grandmother.”