COVID-19 infection after vaccination and what to do next

Vaccines work to dramatically reduce the risk of developing COVID-19, but no vaccine is perfect. Now, with 174 million people already fully vaccinated, a small portion are experiencing a so-called "breakthrough" infection, meaning they test positive for COVID-19 after being vaccinated.

But doctors say this virus -- which can be deadly for an unvaccinated person -- most often leads to much milder symptoms in those who already got their shots, with a recent Centers for Disease Control and Prevention analysis finding vaccinated people are 29 times less likely to require hospitalization and four times less likely to be infected with COVID-19, even when the delta variant is predominant.

“We know vaccination is not 100%," said Dr. Jay Bhatt, an internist and adjunct faculty at the UIC School of Public Health and an ABC News contributor. "That being said, we know that most people in the ICU are unvaccinated individuals.”

Still, breakthrough infections do happen.

The CDC has very specific guidance about what vaccinated people should do if they are exposed to someone with COVID-19, or if they test positive themselves.

If a vaccinated person is exposed to the virus -- meaning a close contact has tested positive -- they don't need to quarantine, but they should get a COVID test three to five days after that exposure. And they should wear a mask in public indoor spaces, like the grocery store, while awaiting test results.

But if a vaccinated person receives a positive test result or has symptoms after exposure, they should isolate for 10 days. A repeat test is not needed at the end of the 10-day isolation period, though the person should be fever-free for at least 24 hours before ending quarantine.

Vaccination status does not change isolation recommendations for those who test positive because they can still be contagious, though the CDC reports that the contagious period may be shorter than in those who are unvaccinated and viral load lessens after five days.

Since those who are vaccinated and infected with COVID-19 are still able to transmit the virus, their close contacts should also be contacted and tested. A close contact is a person who has spent more than 15 minutes with a COVID-19 positive person while unmasked and less than 6 feet apart.

Fully vaccinated people who test positive may also be eligible for authorized COVID-19 treatments, if their doctor says it's necessary. Therapies such as monoclonal antibodies can still be given to COVID-positive patients in a high-risk category, even if they are vaccinated. “Monoclonal antibodies are intended for those with COVID-19 who are high risk, which includes those over 65, and those who have chronic disease, cancer or are immunocompromised," Bhatt said. "The chances of allergic reactions or adverse events are relatively low.”

Monoclonal antibodies, laboratory-made proteins which mimic the immune system’s antibodies, work best when given in the first few days after a positive test result or symptom onset. After receiving monoclonal antibodies, further COVID-19 vaccination, such as a booster, should be delayed by 90 days to optimize response to the vaccine.

Other treatments, such as steroids or antivirals like Remdesivir, are more commonly given for hospitalized and severe cases, which are less likely to occur in vaccinated individuals.

While breakthrough infections are likely to be mild, it’s important to follow recommended guidelines to reduce the spread of infection. Wearing masks in large crowds and staying home when not feeling well will help protect both vaccinated and unvaccinated people.

Priscilla Hanudel, M.D., is an emergency medicine physician in Los Angeles and a contributor to the ABC News Medical Unit.