The Centers for Disease Control and Prevention says it can take weeks for a person’s body to build up immunity after getting vaccinated.

Without masks and a vaccine, we could reach Herd Immunity from COVID-19, but deaths would skyrocket. We break down the science of it.
Its still possible to test positive for the coronavirus even after getting vaccinated, experts said.
Both the Pfizer-BioNTech and Moderna vaccines require two doses per patient to be fully effective. The first Pfizer-BioNTech dose is more than 50% effective in preventing COVID-19, and the second dose increases that protection to about 95%.
The Centers for Disease Control and Prevention said it can take weeks for a persons body to build up immunity after getting vaccinated.
That means its 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.
Nicole Iovine, an infectious disease expert and hospital chief epidemiologist at University of Florida Health, said it takes the average person 10 to 14 days to build up a protective number of antibodies, but each person is different.
Every day that goes by, the chance you get infected goes down a little bit, she said. Any individual person may make an immune response faster or slower than average.
Dr. Richard Zimmerman, a professor of family medicine and public health at the University of Pittsburgh, said it takes a few weeks for a person to build up to the intended level of immunity after the second dose.
Even reaching that threshold doesnt mean a person is 100% immune to the coronavirus. If a vaccine is 95% effective, that means theres still a small chance of infection.
Its not perfect, he said.
Zimmerman said its difficult to pinpoint the time of exposure. A patient could be exposed to the virus before getting vaccinated and display symptoms after. 
Iovine urges those who test positive for the coronavirus to get vaccinated after recovering because the vaccine provides better protection than natural infection.
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“This vaccine gives a very focused and direct immune response to the part of the virus thats necessary to mediate infection,” whereas natural infection triggers an immune response to all parts of the virus, she said. “Oddly enough, it might not be better.” 
The COVID-19 vaccines being used do not have the live virus and wont infect people. Instead, the vaccine teaches the body how to fight the virus.
Experts said its important to continue wearing masks and maintaining social distance until coronavirus circulation is low and the country achieves herd immunity.
If we have around 80% of people immune to the virus, then we think thats when the virus will no longer be able to be transmitted, Iovine said. However, that may change as experts speculate a new variant discovered in the U.K. and South Africa may be more transmissible than the original coronavirus.
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Its a little bit of a moving target, she said.
Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, projects the country may achieve herd immunity by the fall, reaching a degree of normality by the end of 2021.
Though the vaccines may not be 100% effective against COVID-19, Iovine said the risk of infection continues to decrease as more people in a community get vaccinated.
The virus needs to find susceptible people in order to pass around. If it cant find (them), then the chance youre going to contract it is a lot lower, Iovine said. The more people in the community who are vaccinated, the harder it is for the virus to circulate.
Follow Adrianna Rodriguez on Twitter: @AdriannaUSAT. 
Health and patient safety coverage at USA TODAY is made possible in part by a grant from the Masimo Foundation for Ethics, Innovation and Competition in Healthcare. The Masimo Foundation does not provide editorial input.
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