Scientists Are Optimistic That the Vaccine Will Partially Prevent Infection

The data are still out, but scientists say those who are immunized will likely shed less virus than those who aren’t

Photo: Grant Hindsley/AFP/Getty Images

The vaccines are here, they’re safe, and they’re extremely effective at preventing both mild and severe cases of Covid-19. But whether a vaccinated person could still become infected with the SARS-CoV-2 virus and pass it on to other people is another question, one the public and scientists alike are desperate to know the answer to.

Experts say that the virus could still enter cells in a vaccinated person’s nose and mouth and begin to replicate there. The immune response generated by the vaccine would quickly defeat the virus, so the infection wouldn’t last long, and the virus likely wouldn’t be able to get down into the lungs where it can wreak havoc. But the asymptomatically infected person could still unwittingly transmit the virus to other people, especially those who are not yet protected.

A study published earlier this week about the AstraZeneca vaccine, which is not yet approved for use in the United States, provided the first look at data collected explicitly to answer this question. The research showed a 67% reduction in infections after two doses of the vaccine, as well as an 82% drop in cases of symptomatic disease. Other studies that will provide information on the two vaccines currently authorized in the United States, produced by Pfizer/BioNTech and Moderna, are expected in the coming months.

As the vaccine rollout is ramped up across the country, people need guidance on how to behave now, so Elemental asked five scientists and physicians how they expect the vaccines will perform when it comes to transmission. Based on what they know about other vaccines and the data currently available from research in animal models, all six experts said they expect the vaccines will partially prevent the spread of SARS-CoV-2, but they likely will not provide so-called “sterilizing immunity” — complete protection against not only symptomatic disease but also infection with the virus.

The Pfizer/BioNTech and Moderna vaccines offer roughly 95% protection against Covid-19 after two doses. This means that only 5% of people who get both doses will develop respiratory symptoms should they come into contact with the novel coronavirus. That is an incredibly high level of protection, and these vaccines will save hundreds of thousands, if not millions, of lives.

“I think that it’s very unlikely that vaccines this efficacious wouldn’t have some impact on transmission and would not be protective at all against infection,” says Angela Rasmussen, PhD, a virologist affiliated with the Center for Global Health Science and Security at Georgetown University.

“The vaccines that give really high levels of virus neutralizing antibodies are probably also more likely to inhibit virus transmission.”

Up for debate, however, is whether the vaccines protect against infection and transmission.

“You may reduce or eliminate virus transmission to the lungs and other organs and hence disease,” says John Moore, PhD, a professor of microbiology and immunology at Cornell University. “But there’s still virus around and replicating in the nose and upper respiratory tract, which isn’t causing you disease, but you could still sneeze it out on people.”

This possibility is why vaccinated people are still urged to wear masks in public until a large enough swath of the population (estimated at 70%-80%) has been vaccinated to reach herd immunity.

Scientists are optimistic that the vaccine will partially prevent infection

Most scientists think the coronavirus vaccine will land somewhere between the best and worst case scenarios.

“I think that while the vaccine might not completely protect against asymptomatic infection, I predict that those who are immunized will shed less virus than those who aren’t,” says Paul Offit, MD, director of the Vaccine Education Center at Children’s Hospital of Philadelphia.

“I would venture to guess that there would likely be some degree of protection against infection, but it would not be complete,” agrees Yvonne Maldonado, MD, a professor of pediatrics and infectious diseases at Stanford University.

“I have some optimism that we’re going to see a reduction in transmission as well, but it’s still important to do the studies,” adds Peter Hotez, MD, PhD, dean for the National School of Tropical Medicine at Baylor University.

The scientists based their cautious optimism in part on early tests the two vaccine manufacturers performed on macaques, a type of primate. Scientists vaccinated the animals and then exposed them to high levels of the coronavirus. Most — although not all — of the vaccinated macaques showed no signs of viral replication in their nose, mouth, and lung, suggesting the virus was not able to infect them. The vaccinated animals that did become infected had significantly lower levels of virus in their nose and mouth than control macaques that didn’t receive the vaccine.

Even though some of the vaccinated animals still became infected, Rasmussen says the fact that they produced less virus is good news for stopping transmission of the virus because at some point “you’re not actually going to be shedding enough virus to transmit it to other people.”

“You always have to take non-human primate data with a grain of salt. There is no perfect animal model,” she says. “But based on that data, I’d say that I’m optimistic that we’d at least see the same sort of partial protection.”

The amount of antibodies may determine if you can transmit the virus

One potential difference between the vaccinated animals that got infected and the ones that didn’t was the amount of neutralizing antibodies they produced. “The vaccines in nonhuman primates that give really high levels of neutralizing antibodies, they were not shedding virus from their nose and mouth as much. The ones that give low levels, they were still shedding a lot of virus,” says Hotez. Based on this information, he speculates that, “The vaccines that give really high levels of virus neutralizing antibodies are probably also more likely to inhibit virus transmission.”

Data that’s been published from the phase 1 clinical trials show that the antibodies people produce can vary widely, even in response to the same vaccine. This finding suggests that some people may be better protected than others and therefore may have a higher likelihood of stopping transmission.

“Whenever you are in any kind of environment where there might be unvaccinated people, if you’re vaccinated, you should continue to use the same precautions to avoid both exposure risk as well as to prevent transmission risk to others.”

A vaccine protecting against disease but not against infection and transmission is not uncommon. The polio vaccine originally developed by Jonas Salk is one classic example that several of the researchers mentioned. The vaccine can’t stop the virus from infecting cells in the intestines, but it does block the virus from getting into the blood and ultimately invading nerve cells and paralyzing people.

“When you give individuals the polio vaccine, they can get infected [with the virus], but they don’t get paralyzed, they don’t get sick,” Maldonado says. “But they can serve as a source of infectious virus to other people.”

Although they affect the body in very different ways, SARS-CoV-2 and the poliovirus are relatively similar RNA viruses that primarily infect mucosal tissues, namely the respiratory and intestinal tracts. This fact is important because mucosal tissues rely on a different type of antibody (called IgA) to provide protection than the antibodies that circulate in the blood (named IgG).

Rasmussen says that vaccines that are injected, called intramuscular vaccines, “are very, very good at eliciting high levels of IgG, which is the type of antibody that’s most commonly found in the blood, but they’re not as robust at producing potent neutralizing IgA responses. They do produce some, but that may not be enough to provide complete neutralizing protection on that mucosal surface like the nose.” This discrepancy between IgG and IgA antibody levels is why scientists are pursuing intranasal spray vaccines for other respiratory viruses, like influenza, that would have a more immediate impact on mucosal antibodies in the nose.

While the experts remain optimistic that the vaccines will reduce the likelihood of transmission, they all emphasized that we don’t know that for sure yet, and people still need to take precautions, even after they’ve been vaccinated.

“At minimum, until you’ve had some time after the second dose, nothing has changed. You are not protected yourself to a robust extent, in my opinion. And that is something the public needs to bear in mind,” Moore says. “Even after you are ‘fully protected,’ you’ve had this period post second dose, are you still capable of transmitting the virus to someone else? Are you capable of infection at sub-disease level while still being contagious? And that’s where we don’t know the answer. But why don’t we err on the side of caution for a while?”

“Until we do know that with more confidence, I think it makes sense to provide guidance that whenever you are in any kind of environment where there might be unvaccinated people, if you’re vaccinated, you should continue to use the same precautions to avoid both exposure risk as well as to prevent transmission risk to others,” says Rasmussen.

“I think we message that these vaccines are extraordinary, but we don’t know yet what the limitations are, and we just need to wait a little bit longer,” concludes Maldonado. “I’m not saying we should shut down. I do think we should be opening up economies. I just think taking off the masks is not a good idea right now.”

Health and science writer • PhD in 🧠 • Words in Scientific American, STAT, The Atlantic, The Guardian • Award-winning Covid-19 coverage for Elemental

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