Vaccine distribution is increasing in many countries, but with the cases of Covid-19 also growing again and the prospect of another outbreak of infections on the horizon, the world is in a race against time.
The key to winning the race, experts say, is not only whether vaccines will play a significant role in preventing serious illnesses caused by Covid-19, but also preventing people from spreading the virus.
“The ideal vaccine would have two performance characteristics: one prevents you from going to the hospital, going to the ICU and losing your life,” said Dr. Peter Hotez, co-director of the Vaccine Development Center at Texas Children’s Hospital and dean of the School National Institute of Tropical Medicine at Baylor College of Medicine. “But if the vaccine also prevents asymptomatic spread, you can vaccinate your way out of the epidemic.”
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The first indications have been promising so far. The effect of vaccines on asymptomatic infection was largely unknown, but scientists say it will be crucial to ending the pandemic.
Asymptomatic cases involving people infected with Covid-19 but without symptoms are estimated to be responsible for more than half of all transmissions of the virus, according to a recent study published in the journal JAMA Network Open by researchers Centers for Disease Control and Prevention. If vaccines can block asymptomatic infections, they can also significantly reduce overall transmission, offering hope that the virus can be contained soon.
Vaccines can protect against transmission by reducing a person’s viral load, or the amount of virus present in the body, said Dr. Becky Smith, an associate professor of medicine at Duke University.
“Theoretically, by reducing your viral load, this should hinder your ability to transmit to other people,” she said. “And even if it doesn’t completely prevent transmission, it should slow it down significantly.”
The focus on vaccines and transmission comes at an important time in the pandemic. Although cases globally have dropped for several weeks, some European countries are now seeing a recovery. Parts of the US are also reporting increases, a worrying development, as many states have recently relaxed public health restrictions.
Concerns about coronavirus variants, including strains that may be more contagious, also persist. The government’s leading infectious disease expert, Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, told Richard Engel of NBC News on Thursday that the United States needs to vaccinate as many people as possible to prevent further outbreaks .
Part of that strategy depends on the effect that the vaccine can have on reducing transmission.
Last week, new data from Israel, where nearly 60 percent of the country’s 9 million residents received at least one dose of a vaccine, suggested that the Pfizer-BioNTech vaccine is 94 percent effective in preventing asymptomatic infections.
A separate study conducted by researchers at the University of Cambridge, UK, found that a single dose of the Pfizer vaccine can reduce asymptomatic infections by 75 percent. The results, which have not yet been peer-reviewed, came from an analysis of some 4,400 tests performed on vaccinated health professionals in Cambridge over a two-week period in January.
In Johnson & Johnson’s tests, the company’s vaccine was found to be 74% effective against asymptomatic infections. And according to a report released in December by the Food and Drug Administration, early data suggested that Moderna’s vaccine may also protect against asymptomatic infections, but the company said more research is needed.
Angela Rasmussen, a virologist at the Center for Global Health Science and Safety at Georgetown University, said the first findings are “very promising”. But she added that there are still some big unanswered questions.
“From the real-world data we have so far, it appears that vaccines have an impact on asymptomatic infection,” she said. “The real question, however, is how wide will it be?”
And since vaccines are not 100 percent effective, it is possible that a small number of people vaccinated could be infected with the virus. If that happens and a vaccinated individual is asymptomatic, it remains to be seen whether the person can transmit Covid-19 to others, said Rasmussen.
In a new comment published on Thursday in Science magazine, Rasmussen and Saskia Popescu, an infectious disease epidemiologist at George Mason University in Virginia, detail why controlling “symptom-free transmission” is critical to ending the pandemic. Symptom-free transmission includes both people who have no symptoms and those who are pre-symptomatic, but later develop symptoms.
“As more people are vaccinated, this will have a transmission effect on the entire population, but although most people are now not vaccinated, we need to be aware of the issue of asymptomatic and pre-symptomatic transmission,” said Rasmussen.
Most scientists agree that there are two main ways out of the pandemic. One involves reaching a limit known as herd immunity – when enough people have developed antibodies to natural infection or vaccines, future outbreaks are unlikely. The other requires controlling the spread of the virus in such a way that even unvaccinated parts of the population are at little risk of being infected.
If vaccines can protect against asymptomatic infection, they can help with the latter, but the two strategies should not be mutually exclusive, said Rasmussen.
“It is really a set of interventions,” she said. “We need to think of ways to decrease transmission in general and not rely solely on vaccines.”
One way to reduce general transmission is to pay attention to public health measures in place during the pandemic, such as practicing social detachment, wearing masks and avoiding encounters with unvaccinated people. If the virus can be adequately contained, aspects of life can return to normal, even if parts of the population have not yet been vaccinated, said Rasmussen.
“We don’t need to be on the herd’s immunity limit to relax restrictions,” she said. “If we get the virus to be so uncommon in the population, there is no risk that people will be exposed to it, whether they are vaccinated or not.”