Two people could walk into the same pharmacy on the same day and receive the same coronavirus vaccine – but their side effects will likely differ.

That's because each person's immune system is unique. How it responds to vaccines depends on broad categories like age and sex, as well as more individual characteristics like our genes or history of exposure to infections.

"Your immune response essentially dictates your side effects," Dr Vivek Cherian, an internal-medicine physician in Baltimore, told Insider.

In general, women and younger adults tend to have the most severe side effects after their coronavirus shots, no matter the dose or manufacturer.

For all three US-authorized coronavirus vaccines – from Pfizer, Moderna, and Johnson & Johnson – younger adults more commonly reported fatigue, headache, and pain at the injection site than older adults did.

Side effects are also more numerous and severe after dose two of Pfizer's or Moderna's shot.

Adults over 65 generally have weaker immune systems

Our immune systems tend to deteriorate as we age, so older people's bodies don't work as hard to defend them against foreign invaders – including the protein introduced to the body via a vaccine. As a result, side effects are often milder and less numerous among the elderly than among younger adults.

"Usually when we say elderly, we're referring to individuals over 65," Cherian said.

After one dose of Moderna's shot, 57 percent of people younger than 65 developed side effects, compared with 48 percent of those older than 65.

After the second dose, nearly 82 percent of people in the younger group developed side effects, compared with nearly 72 percent of older adults.

Pfizer also reported that fatigue, headaches, and pain at the injection site were more common among people ages 18 to 55 after both doses than among people ages 56 and up.

In Johnson & Johnson's trial, nearly 62 percent of people ages 18 to 59 developed side effects, compared with 45 percent of people ages 60 and up.

But that doesn't mean vaccines are any less effective for the older group.

"For the COVID-19 vaccine, we've actually not seen decreased effectiveness as we get older, so that's actually a really good thing," Cherian said.

Estrogen plays a role in a woman's immune response

Women tend to have stronger reactions to many vaccines, including those for polio, influenza, measles, and mumps – so it's no surprise that they have more side effects after their coronavirus shots, too. Cherian said this likely has to do with estrogen levels.

"Testosterone tends to be an immune-suppressive hormone, and estrogen tends to be an immune stimulant," he said.

A recent report from the Centers for Disease Control and Prevention found that roughly 79 percent of instances of vaccine side effects reported to the CDC came from women, though just 61 percent of doses were administered to women overall.

Vaccine side effects are even more pronounced for pre-menopausal women than post-menopausal woman, Cherian said.

The second dose produced stronger side effects across age groups

On Monday, the CDC released a report that examined side effects among 1.9 million Americans who'd received both doses of Pfizer's or Moderna's vaccines. Overall, roughly 50 percent of vaccine recipients reported side effects after their first dose, while 69 percent reported side effects after their second shot.

Across the board, reports of injection-site pain rose from 68 percent after dose one to 72 percent after dose two. Fatigue rose from 31 percent to 54 percent from the first to second shot, headaches from 26 percent to 47 percent, and body or muscle aches from 19 percent to 44 percent.

Nearly 75 percent of Moderna recipients had side effects after dose two, compared with 64 percent of Pfizer recipients. These results were generally consistent with those from clinical trials.

More side effects after the second dose is generally a sign that your body has learned to quickly recognize the viral protein and is ready to attack it again.

"I can't emphasize enough the importance of not delaying the second dose, because you don't get full efficacy until you get that second dose," Cherian said.

Indeed, a CDC report found that a single dose of either Moderna's or Pfizer's vaccine was 80 percent effective in preventing coronavirus infections after two weeks. Two weeks after the second dose, the vaccines' effectiveness jumped to 90 percent.

This article was originally published by Business Insider.

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