Stories like this will become more common as millions of Americans are administered the Pfizer-BioNTech and Moderna vaccines over the coming months. Over time, many who are vaccinated will still get infected with the novel coronavirus.
During trials, the vaccines were shown to be about 95% effective — which means some vaccinated people were still infected.
It takes time for vaccines to build up immunity, and the two authorized coronavirus vaccines both require two doses, given several weeks apart, to train the body’s immune system. People can be exposed to coronavirus right before being vaccinated, or right after, and there won’t be time for the body to develop its defenses.
“That means it’s possible a person could be infected with the virus that causes Covid-19 just before or just after vaccination and still get sick,” the CDC says.
Vaccines might not provide perfect protection
No vaccine is 100% effective, and the makers of coronavirus vaccines are still evaluating whether the shots protect against all infections, or just those that cause symptoms.
The CDC estimates that 40% of coronavirus infections don’t cause symptoms, and the trials of both the Moderna and Pfizer/BioNTech vaccines looked only at whether the vaccines prevented symptomatic infections.
Moderna said in December it had submitted data to the US Food and Drug Administration showing its vaccine prevented 2/3 of all infections, including asymptomatic infections. For now, the CDC recommends that people not assume they are completely immune to infection after having been vaccinated.
Overall, both vaccines provided about 95% protection in clinical trials — which means 5% of people might still catch the virus even after two shots. In wider use, this efficacy rate may go down as people with varying levels of immune system response get vaccinated and then go out into the world.
It’s not because the vaccine gave you the virus
The current coronavirus vaccines cannot infect anyone with the virus. They don’t contain the virus.
Immunity may wane over time
No one knows how long the vaccines out now will protect people from infection.
Coronavirus has only been around for about a year, and the final phases of testing the vaccines only wrapped up a few weeks ago. Pfizer and Moderna followed volunteers for at least two months after their second doses.
There is also the possibility that the novel coronavirus might mutate in a way that makes the vaccines less effective. Influenza virus strains mutate constantly and that’s one reason people need fresh flu vaccines every year.
Doctors hope coronavirus won’t mutate like flu does. If that does happen, however, the technology used to make the new coronavirus vaccines is designed to be easily adapted. It should take far less time to update the Moderna and Pfizer vaccines than it takes to make new flu vaccines.
Others might not be safe from you
That’s because even people who are themselves immune to the virus might be exposed to it and transmit it to others. It can grow in the nose, says Fauci, director of the National Institute of Allergy and Infectious Diseases.
“It’s possible that someone could get the vaccine but could still be an asymptomatic carrier,” said CNN Medical Analyst Dr. Leana Wen, an emergency physician. “They may not show symptoms, but they have the virus in their nasal passageway so that if they’re speaking, breathing, sneezing and so on, they can still transmit it to others.”
Given these unanswered questions, the CDC says vaccinated people should still use “all the tools available to us” to stop the pandemic, including wearing a mask and staying at least 6 feet away from others.
CNN’s Elizabeth Cohen and Faye Chiu contributed to this report.