(NEXSTAR) - Headache, chills, fever -- these are just a few of the side effects some have reported after getting the Moderna or Pfizer COVID-19 vaccines.
Why do some people experience more side effects from the vaccine than others?
We don't quite know, says Stanford University infectious disease physician Anne Liu.
What we do know are trends, and typically, women and younger people tend to report a greater intensity of side effects than older and male individuals.
"That's not to say they have more symptoms, however," Liu said. It's possible that younger folks and women just report their symptoms with more frequency.
"I'm going to make a big generalization here, but our culture typically discourages men from bringing their symptoms to attention," Liu said.
As for why younger people experience more symptoms, that could be a result of them having stronger immune systems.
"Older people's immune systems tend to get weaker over time," Liu explained, meaning their reaction to the vaccine will be less intense.
"If you have no side effects, you may have a weaker response," Liu said.
That's not to say the vaccine isn't working. The immune system is complex and multi-pronged, and side effects aren't the only sign the vaccine is efficacious.
"Overall, the vaccines are very effective in all groups studied," Liu said.
Both the Pfizer and Moderna vaccines are about 95-percent effective at preventing COVID-19 infection, the drug makers have said.
The matter of vaccine side effects prompts a second question: Why does the second dose of the COVID-19 vaccine appear to have more symptoms than the first dose?
According to Dr. William B. Greenough III, a professor emeritus at Johns Hopkins School of Medicine who’s known for his work on cholera, says there’s a simple explanation for the increase in side effects.
With the first dose, your body begins building its initial immune response, including producing antibodies.
But with the second shot — a.k.a. the second exposure to the virus — “the big guns” of your immune system react.
“Once your immune system recognizes the virus [after the second dose], it’s going to have a greater reaction to it,” Greenough said.
This is a sign the vaccine is working and that your immune system “recognizes the vaccine in your body.”
With the second dose, your body not only creates antibodies, but spurs your lymph nodes and other parts of the body to react to the vaccine at the tissue level, hence additional side effects.
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