The goal of Covid-19 vaccines is to prevent death and serious health complications that plague our overburdened healthcare system. All vaccines licensed for emergency use do, and their safety and efficacy in clinical trials has exceeded expectations. But most people, understandably, want to know 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 for sure that vaccines prevent the virus from being passed from person to person. This can be misinterpreted as an admission that vaccines don’t work. This is not the case. The limited data available suggests that vaccines will at least partially reduce transmission, and studies to determine this with more clarity are ongoing. There should be more data in the coming months. Until then, precautionary measures such as masking and distancing from the presence of unvaccinated people will remain important.
It is true that, according to clinical trial data, the Pfizer-BioNTech and Moderna vaccines are very effective in preventing Covid-19, the disease, but it is not known to what extent they prevent SARS-CoV infection. 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 is possible that vaccinated people themselves are protected against Covid-19, but continue to transmit SARS-CoV-2 to others who are not vaccinated.
Why would scientists make vaccines that only protect against a disease rather than the virus that causes it? They don’t intend to do it, but that’s partly the result of the requirements of clinical trials. In practice, clinical trials can be completed faster if the endpoint of the trial – the primary scientific question the study examines – is something that can be easily observed. If SARS-CoV-2 infection was the endpoint of the trial, clinical trial participants should be tested at least once a week. It’s easier to identify participants who develop symptoms of Covid-19, then dab them to confirm. So, for the sake of effectiveness, the main endpoint in clinical trials was whether vaccines protect against symptoms of Covid-19.
This study approach also makes sense from a public health perspective. Most people infected with SARS-CoV-2 will not die, but many will become very ill and require medical attention. It fills the hospitals and puts tremendous strain on the healthcare system. Vaccines that can turn what would normally be a serious illness into something light and manageable relieve that burden, saving lives and improving their quality.
When scientists develop a vaccine against a new virus, it is difficult to predict whether the vaccination will completely prevent infection – called sterilizing immunity. If the Covid-19 vaccines do not provide sterilizing immunity, it means that a vaccinated person can still inhale enough of the SARS-CoV-2 virus to develop an infection, and it will be quickly cleared from the body before it becomes Covid-19. , but that person could still pass the infection on to another person.
There are many vaccines which do not provide fully sterilizing immunity, but which nevertheless have enormous public health benefits. Every year, the flu shot saves lives and keeps people out of hospital despite not completely preventing infection.
From everything we know so far, it is highly unlikely that vaccines that are 95% effective in preventing symptomatic disease will have no impact on infection. Data from animal studies and vaccine trials suggest that vaccination reduces asymptomatic infection, as well as the amount of virus produced in those infected. In Israel, where a significant portion of the population has been vaccinated, there has been a significant drop in cases since vaccination began in December, with a 49% decrease seen in people over 60, according to a preliminary report. Studies to better determine the impact of vaccines on transmission are underway, and in the meantime, if precautions like masking are combined with increasing vaccination, SARS-CoV-2 cases are expected to drop.
Historical evidence shows that vaccines that do not prevent viral infection can still stop epidemics in their tracks. The polio vaccine developed by Dr. Jonas Salk, which does not provide sterilizing immunity, led to the rapid elimination of polio in the United States from the 1950s. People were queuing impatiently to receive the vaccine to protect themselves and their children. The Salk vaccine was very protective against the devastating effects of the disease and also helped reduce the spread of the virus, as so many people were vaccinated and could clear their infection.
These Covid-19 vaccines are as much a victory for public health today as the Salk vaccine was then. We would do well to remember the transformative power of vaccines that prevent disease without completely preventing infection when enough people get vaccinated. The sooner we reduce the spread in the community and protect as many people as possible with vaccination, the sooner we can relax.
Angela L. Rasmussen is a virologist at the Center for Global Health Science and Security at Georgetown University Medical Center. She is studying the host’s response to infection with emerging viruses, including the coronavirus.
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