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January 06, 2021

Some people are testing positive for COVID-19 after being vaccinated, but that's expected

Protection doesn't begin until 10 to 14 days after receiving the first dose, experts say

Prevention COVID-19
Vaccine COVID-19 Positive Thomas P. Costello/USA Today Network

Mitigation efforts, such as mask wearing and social distancing, will continue to be important even as vaccinations take place. Above, Stana Leigh Ziemba, a nurse with the Monmouth County Health Department in New Jersey, draws a dose of the Moderna COVID-19 vaccine into a syringe.

Reports of health care workers contracting COVID-19 after receiving their first dose of the Moderna or Pfizer vaccines have raised concerns about the efficacy of the vaccines. But experts say these cases aren't surprising.

A Texas home health care service office manager received her first dose of the Moderna COVID-19 vaccine on Christmas Eve. A week later she tested positive for the coronavirus. In between she attended a holiday gathering of eight people.

A Californian emergency room nurse said also he tested positive for COVID-19 after being inoculated. He got his first dose Dec. 18 and developed symptoms six days later — after working a shift in a COVID-19 unit at one of the two hospitals he serves.

Infectious disease experts say these types of cases are expected, with the exposures most likely occurring right before or after they received their first dose. That's why following mitigation efforts such as mask wearing, social distancing and hand washing will continue to be important as the vaccination process plays out. 

So why do these cases occur? 

First, it is important to remember that neither the Pfizer nor the Moderna vaccines offer 100% protection against the coronavirus. During clinical trials, the Pfizer vaccine was 95% effective and the Moderna vaccine was 94% effective.

Second, that protection doesn't kick in immediately after the first dose. 

On average it takes 10 to 14 days to develop enough antibodies for protection, but that the timing can vary by person, Dr. Nicole Iovine, an infectious disease expert at University of Florida Health, told USA Today.

The second dose is needed for maximum effect.

"That first does we think gives you somewhere around 50% (protection), and you need that second dose to get up to 95%," Dr. Christian Ramers, an infectious disease specialist with Family Health Centers of San Diego, told ABC

Even after receiving the second dose, it could take weeks for the immune system to build a strong enough defense, according to The Centers for Disease Control and Prevention.

"That means it's possible a person could be infected with the virus that causes COVID-19 just before or just after vaccination and get sick," the agency said. "This is because the vaccine has not had enough time to provide protection."

So what can you do to better protect yourself and others from the spread of COVID-19? 

Even after being vaccinated, experts urge people to continue following the public health measures that have been put in place to reduce community spread of the coronavirus.

Vaccinated people not only have a small risk of becoming infected, but it's possible they may still spread the virus. Clinical trials demonstrated the vaccines prevent illness — it remains unknown whether they prevent transmission. 

Hand washing, face masks and social distancing will continue to be necessary until herd immunity is established. About 80% of the country will need to be vaccinated before that happens, experts say.

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