Can vaccinated people still spread COVID-19?


When the U.S. Centers for Disease Control and Prevention Changed the guidelines on wearing masks On May 13, many Americans were a little confused. Now, anyone who is fully vaccinated can participate in large and small indoor and outdoor activities without the need to wear a mask or maintain physical distance.

Anthony Fauci, President Biden’s chief medical adviser, said the new guidelines are “Science-based development“And “as an incentive” Nearly two-thirds of Americans People who have not been fully vaccinated can continue to vaccinate.

But some people Cannot be vaccinated Because of the basic conditions. Other people with weakened immune systems, due to cancer or medications, May not be adequately protected Through their vaccinations. Children aged 12 to 15 are eligible The Pfizer-BioNTech vaccine will only be available on May 10.with No COVID-19 vaccine has been approved For near 50 million children Under 12 years old in the United States.

As restrictions were lifted, people began to keep masks at home, and some people were worried: Will you get COVID-19 from someone who has been vaccinated?

Vaccines don’t always prevent infection

Researchers had hoped Design a safe COVID-19 vaccine That would Prevent at least half The number of people vaccinated due to symptoms of COVID-19.

Fortunately, the vaccine has Greatly Outperform expect. E.g, Among the 6.5 million inhabitants of Israel, 16 years old and above, Pfizer COVID-19 vaccine was found to be 95.3% effective after two injections. Within two months, among the 4.7 million fully vaccinated people, the detectable infection rate dropped by a factor of 30.Same in California with Texas, Only 0.05% of fully vaccinated healthcare workers tested positive for COVID-19.

Vaccine developers often hope that in addition to disease prevention, their vaccines can also achieve “Bactericidal immunity“Vaccinations even completely prevent bacteria from entering the body. This bactericidal immunity means that the vaccinated person will neither infect the virus nor spread the virus further. However, for the vaccine to be effective, it does not need to prevent bacterial infections. People.

This Salk inactivated polio vaccine, E.g, Did not stop completely The polio virus grows in the human intestine.but it very effective Prevent disabling diseases because it triggers antibodies to prevent the virus from infecting the brain and spinal cord.Ok Vaccines provide effective and durable training For the body’s immune system, when it actually encounters a pathogenic pathogen, it is ready to respond optimally.

When it comes to COVID-19, immunologists are still figuring out what they call “Relevance of protection,” the factor predicting the degree of protection of someone from the coronavirus. Researchers believe Optimal quantity of”Neutralizing antibody,” The type that can not only bind to viruses but also prevent infection, it is enough to resist Repeat infection. Scientists are still evaluating Immunity Persistence COVID-19 vaccine is being provided and Where in the body It is working.

Can vaccinated people spread the coronavirus?

Immunologists hope that vaccines to prevent viral diseases can also reduce the spread of the virus after vaccination. However, it is actually difficult to determine whether the vaccinated person will not spread bacteria.

COVID-19 presents a special challenge, because asymptomatic and pre-symptomatic people can spread the disease-and insufficient contact tracing and testing means that those people are asymptomatic Rarely foundSome scientists estimate the number of asymptomatic COVID-19 infections in the total population May be 3 to 20 times higher Exceed the number of confirmed cases.Studies have shown that undocumented cases of COVID-19 in people who are asymptomatic or suffering from very mild illness may be responsible for Up to 86% of all infections, Although other studies Contradicts high estimates.

in A study, CDC conducts coronavirus infection tests on volunteer medical staff and other frontline workers in eight locations in the United States every week for three months, regardless of symptoms or vaccination status. Researchers found that fully immunized participants were 25 times less likely to test positive for COVID-19 than unvaccinated participants.Findings like this mean that if vaccinated people are completely immune to infection, they Not likely to spread virus. But without contact tracing to track spread among larger populations, it is impossible to know whether this assumption is correct.

What we can be sure of is that if someone does contract COVID-19 after being vaccinated, the so-called “breakthrough infection”, Symptoms will be milderThe study found that people who tested positive after contracting COVID-19 Just their first dose of vaccine Have They have low levels of virus in their bodies Compared with people who have not been vaccinated but have tested positive.Researchers believe that the reduction in viral load indicates that the vaccinated person is indeed infected with the virus. Less infectious Because they can spread far fewer viruses to others.

A preprint study that has not been peer-reviewed shows that the Moderna COVID-19 vaccine can produce anti-coronavirus Antibodies in oral and nasal fluidsSince This is where the coronavirus comes in, The antibodies in the nose and mouth should prevent the virus from entering the body and effectively provide “bactericidal immunity”. This also means that the vaccinated person may not spread the virus through respiratory droplets.

The evidence is promising.But without more research, scientists Can’t conclude yet The COVID-19 vaccine can indeed prevent all spread. Research attempt Answering this question directly through contact tracing has just begun: Researchers will track COVID-19 infections between vaccinated and unvaccinated volunteers and their close contacts.

Protection and prevention go hand in hand

Vaccines help slow the spread of infectious diseases by breaking the chain of infection. Those who are infected eventually spread the virus to fewer and fewer unprotected people.This is how the vaccine increases Herd immunity – People who are susceptible and not yet immunized are surrounded by a group of people who have gained immunity from vaccinations or previous infections.But research shows that for biology and Social Causes, Vaccination alone is unlikely In order to achieve herd immunity against COVID-19 and complete control of the coronavirus.

In fact, vaccination It may take a long time for one person Eradicate any disease. Even diseases that are almost “eliminated”—such as chickenpox, measles, and whooping cough— Can resurface As immunity declines and vaccination rates decline.

Recent outbreak of infection Vaccinated New York Yankees It shows that not only the vaccinated people may still be infected, they may also spread the coronavirus to close contacts. Highly tested groups such as professional sports teams emphasize the fact that vaccinated mild, asymptomatic infections in the general population may actually be more frequent than reported.A similar Singapore airport staff broke out Shows that even in the case of full vaccination, new and more infectious mutations can spread quickly.

The CDC’s lax guidelines on wearing masks are designed to reassure vaccinated people that they will not contract serious diseases. they are. But for the unvaccinated people who interact with them, the situation is less clear.Before achieving near-herd immunity against COVID-19 and accumulating clear evidence that vaccinated people will not spread the virus, I and Many epidemiologists I believe it is best to avoid situations that may be infected.Vaccination and Continue to cover Maintaining social distancing is still an effective way to stay safe.


Sanjay Mishra, Project Coordinator and Staff Scientist, Vanderbilt University Medical Center, Vanderbilt University

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