TOPEKA, Kan. (NEXSTAR) – It’s widely known that the second dose of the Pfizer and Moderna COVID-19 vaccines come with more side effects, including headache, fever and chills.

Even then, you shouldn’t skip it, says Dr. Anne Liu, an infectious disease physician at Stanford University.

“For the two-dose vaccines, we know that the first dose is at least partially protective and probably significantly so, but we only have data for the short term — a month or two from receiving the vaccine,” Liu said.

“We don’t really know what happens after that. It’s possible you need the second dose to create a longer-lasting effect.”

Variants pose a “theoretical concern” as well. It’s been hypothesized that receiving just one dose of the two-shot vaccines may increase the chance for a variant to arise, but this is just hypothetical at the moment, Liu said.

“I don’t think we’re seeing [the rise of variants from single doses] much yet,” Liu said. “We’ll see as virus cases go back up if that’s the case.”

“The vaccines were not studied in a one-dose regiment, so we don’t really know yet,” she added. “But I’m worried about it.”

If someone is fearful of experiencing second-dose symptoms, Liu recommends seeking out the single-dose Johnson & Johnson vaccine, which has fewer reported side effects.

Why do some people experience more side effects from the vaccine than others? 

We don’t quite know, Liu says. 

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.