On December 18, a San Diego emergency room nurse was given a shot of the COVID-19 vaccine. A week later, he tested positive for the virus, CNN affiliate KGTV reported.

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.

But the vaccines have been shown to be 95% effective, so how can this happen?

Immunity Doesn't Kick in Right Away

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.

The Centers for Disease Control and Prevention says building immunity "typically takes a few weeks."

"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.

The 95% efficacy number for the COVID-19 vaccines also assumes some built-in wait time. Moderna measured the efficacy of its vaccine  after the second dose, while Pfizer measured it starting seven days after the second dose.

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Note:
Thu, Jan 21, 8:30p Port Of Harlem contributor Dr. Theo Hodge, Jr. joins Port Of Harlem Talk Radio Thursday, January 21 to talk about the similarities of dealing with HIV and COVID-19 education and pandemics in one lifetime. He is actively involved in multiple speaker bureaus dedicated to educating healthcare providers in the management of the HIV infected.