World Views | Can a vaccinated person spread coronavirus?

As restrictions are lifted and people start to leave their masks at home, some people worry: Can you catch COVID-19 from someone who’s vaccinated?
Researchers had hoped to design safe COVID-19 vaccines that would prevent at least half of the people vaccinated from getting COVID-19 symptoms.
Fortunately, the vaccines have vastlyoutperformed expectations. For example, in 6.5 million residents of Israel, aged 16 years and older, the Pfizer–BioNTech mRNA COVID-19 vaccine was found to be 95.3% effective after both shots. Within two months, among the 4.7 million fully vaccinated, the detectable infections fell by 30-fold. Similarly in California and Texas, only 0.05% of fully vaccinated health care workers tested positive for COVID-19.
Vaccine developers often hope that, in addition to preventing illness, their vaccines will achieve “sterilizing immunity,” where the vaccination blocks the germ from even being able to get into the body at all. This sterilizing immunity means someone who’s vaccinated will neither catch the virus nor transmit it further. For a vaccine to be effective, though, it doesn’t need to prevent the germ from infecting an immunized person.
The Salk inactivated polio vaccine, for instance, does not completely stop polio virus from growing in the human gut. But it is extremely effective at preventing the crippling disease because it triggers antibodies that block the virus from infecting the brain and spinal cord. Good vaccines provide effective and durable training for the body’s immune system, so when it actually encounters the disease-causing pathogen, it’s ready to mount an optimum response.
When it comes to COVID-19, immunologists are still figuring out what they call the “correlates of protection,” factors that predict just how protected someone is against the coronavirus. Researchers believe that an optimum amount of “neutralizing antibodies,” the type that not only bind the virus but also prevent it from infecting, are sufficient to fend off repeat infections. Scientists are also still assessing the durability of immunity that the COVID-19 vaccines are providing and where in the body it’s working.
Immunologists expect vaccines that protect against viral illnesses to also reduce transmission of the virus after vaccination. But it’s actually tricky to figure out for sure if vaccinated people are not spreading the germ.
COVID-19 poses a particular challenge because people with asymptomatic and pre-symptomatic infections can spread the disease – and insufficient contact tracing and testing mean those without symptoms are rarely detected. Some scientists estimate that the number of asymptomatic COVID-19 infections in the overall population could be 3 to 20 times higher than the number of confirmed cases.
In one study, the CDC tested volunteer health care personnel and other front-line workers at eight U.S. locations for SARS-CoV-2 infections weekly for three months, regardless of symptoms or vaccination status. The researchers found that fully immunized participants were 25 times less likely to test positive for COVID-19 than were those who were unvaccinated. Findings like this imply that if vaccinated people are so well protected from getting infected at all, they are also unlikely to spread the virus. But without contact tracing to track transmission in a larger population, it’s impossible to know if the assumption is true.
What we know for sure is that if someone does get sick with COVID-19 after vaccination, in what is called a “breakthrough infection,” symptoms will be milder. Studies have found that people who tested positive for COVID-19 after getting just their first vaccine dose had lower levels of virus in their bodies than unvaccinated people who tested positive. The researchers believe the decreased viral load hints that vaccinated people who do contract the virus will be less infectious because they will have much less virus that could be spread to others.
Vaccines help slow down the spread of an infectious disease by breaking the chain of infection.
Vaccination coupled with continued masking and social distancing is still [the most] effective way to stay safer. Protection and prevention go hand in hand. Sanjay Mishra, Vanderbilt University MDT/The Conversation
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