The purpose of the Covid-19 vaccines is to prevent death and serious health complications that strain our overburdened health care system. All the vaccines authorized for emergency use do this, and their safety and effectiveness in clinical trials have surpassed expectations. But most people, quite understandably, want to know something more: Will being vaccinated stop the spread of Covid-19 so they can socialize outside their bubbles and dine indoors with abandon?
Many scientists are reluctant to say with certainty that the vaccines prevent transmission of the virus from one person to another. This can be misinterpreted as an admission that the vaccines do not work. That’s not the case. The limited data available suggests the vaccines will at least partly reduce transmission, and the studies to determine this with more clarity are underway. There should be more data within the next couple of months. Until then, precautionary measures like masking and distancing in the presence of unvaccinated people will remain important.
It is true that, according to the clinical trial data, both the Pfizer-BioNTech and Moderna vaccines are highly effective at preventing Covid-19, the disease, but it’s unknown how well they prevent infection with SARS-CoV-2, the virus. Although Covid-19 and SARS-CoV-2 are often used interchangeably, they are fundamentally different. You can’t have the disease without the virus, but you can have the virus without the disease — as many asymptomatic people already know. It’s possible that vaccinated people are protected against Covid-19 themselves, but still spread SARS-CoV-2 to others who are not vaccinated.
Why would scientists make vaccines that protect against only a disease rather than the virus that causes it? They don’t set out to do that, but it is the result, in part, of the exigencies of clinical trials. Practically, clinical trials can be completed more quickly if the endpoint of the trial — the main scientific question the trial is investigating — is something that can be easily observed. If SARS-CoV-2 infection were the trial endpoint, participants in the clinical trials would need to be tested at least weekly. It’s easier to identify participants who develop Covid-19 symptoms and then swab them to confirm. So for efficiency’s sake, the primary endpoint of the clinical trials was whether the vaccines protect against Covid-19 symptoms.
This study approach also makes sense from a public health perspective. Most people who are infected with SARS-CoV-2 will not die, but many will become very sick and require medical care. This fills up hospitals and places significant strain on the health care system. Vaccines that can transform what would normally be a severe illness into something mild and manageable relieve this burden, saving lives and improving their quality.
When scientists develop a vaccine against a novel virus, it’s difficult to predict whether vaccination will completely prevent infection — what’s called sterilizing immunity. If the Covid-19 vaccines do not provide sterilizing immunity, it means a vaccinated person can still inhale enough of the SARS-CoV-2 virus to develop an infection, and it will be swiftly cleared from the body before becoming Covid-19, but that person could still pass the infection to another person.
There are many vaccines that do not provide fully sterilizing immunity but nonetheless have huge public health benefits. Every year, the flu vaccine saves lives and keeps people out of the hospital despite the fact that it doesn’t prevent infection altogether.
From everything we know so far, it’s highly unlikely that vaccines that are 95 percent effective at preventing symptomatic disease would have no impact whatsoever on infection. Data from animal studies and vaccine trials suggests that vaccination reduces asymptomatic infection, as well as the amount of virus produced in people infected. In Israel, where a substantial portion of the population has been vaccinated, there has been a significant decline in cases since vaccination began in December, with a 49 percent decrease observed in people over age 60, according to a preliminary report. Studies to better determine the impact of vaccines on transmission are ongoing, and in the meantime, if precautions like masking are paired with increasing immunization, SARS-CoV-2 cases should plummet.
Historical evidence shows that vaccines that do not prevent virus infection can still stop epidemics in their tracks. The polio vaccine developed by Dr. Jonas Salk, which does not provide sterilizing immunity, resulted in the rapid elimination of polio in the United States beginning in the 1950s. People lined up eagerly to receive the vaccine to protect their children and themselves. The Salk vaccine was highly protective against the devastating impact of the disease and also worked to reduce spread of the virus because so many people were vaccinated and could clear their infection.
These Covid-19 vaccines are as much a victory for public health now as the Salk vaccine was then. We would do well to remind ourselves of the transformative power of vaccines that prevent disease without completely preventing infection when enough people take the vaccine. The sooner we reduce spread in the community and protect as many people as possible through vaccination, the sooner we’ll be able to relax.
Angela L. Rasmussen is a virologist at the Center for Global Health Science and Security at Georgetown University Medical Center. She studies the host response to infection with emerging viruses, including the coronavirus.
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