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People line up outside an immunization clinic to get their Oxford-AstraZeneca COVID-19 vaccine in Edmonton on April 20, 2021.JASON FRANSON/The Canadian Press

Candice Arnold says she’s never felt so happy about feeling so terrible. This week, as soon as people 40 and older became eligible for the AstraZeneca COVID-19 vaccine, the Winnipeg resident quickly booked an appointment and received her first dose.

About eight hours later, the side effects started. She felt tired and achy, as though she’d had a tough workout. Another two hours later, she woke up feeling like her head was in a vise. She was feverish, both chilly and hot, and lay in bed whimpering. The effects lasted about 20 hours, then she felt back to her old self.

It was worth it, Ms. Arnold says, saying she felt reassured the vaccine was working to produce an immune response. But not all her friends experienced that.

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“Some of them had the exact same reaction,” she says. “Others were like, ‘Oh, my arm’s a little sore.’ ”

Why do some people have what’s been described as a “vaccine hangover,” while others feel none at all? We asked experts to weigh in.

Why do some people have worse side-effects?

Younger people tend to have worse side effects than older adults, since our immune responses become less robust with age, says Jason Kindrachuk, assistant professor in the department of medical microbiology and infectious diseases at the University of Manitoba.

Women also seem to have more side effects than men. However, Dr. Kindrachuk says there’s not a clear answer yet as to why. Some researchers are examining how different sex hormones may play a role in differential immune responses between men and women, he says, while others are studying potential behavioural differences.

The important thing to note is that the vaccine’s effectiveness isn’t tied to how strong your reaction is, says Kelly Grindrod, a pharmacist and associate professor at the University of Waterloo School of Pharmacy.

“You don’t need to have side effects for the vaccine to work,” she says, saying all four approved vaccines – Pfizer, Moderna, AstraZeneca (and its counterpart Covishield) and Johnson & Johnson – work well, regardless of age.

How do the common side effects from the AstraZeneca (and Covishield) vaccine compare with those from mRNA vaccines by Pfizer and Moderna?

They’re quite similar, Dr. Grindrod says. Lately, people may be hearing more about side effects from the AstraZeneca vaccines because more younger people are getting them, now that they’re eligible, she says. Older people who received other vaccines are less likely to experience side effects in general, and front-line workers who were vaccinated during an earlier stage of the rollout are fewer in number, and perhaps less likely to share their experiences on social media, she says.

With AstraZeneca, about one-third of people have fever, two-thirds get fatigue, about a third experience joint pain, a little less than two-thirds get headaches and about a quarter have nausea, Dr. Grindrod said, saying this is about the same as for the mRNA vaccines. With the latter, however, the side effects tend to be worse after the second dose, whereas with AstraZeneca, they appear to be worse after the first dose, she says.

Bottom line: Don’t hold off for a different vaccine. Differences in how people respond have more to do with individual genetic differences than differences in vaccines, says Jorg Fritz, associate professor in the department of microbiology and immunology at McGill University.

Is it safe to take over-the-counter medications to alleviate discomfort?

“Absolutely,” Dr. Grindrod says, saying that even during vaccine trials, participants had the option of taking pain relievers, such as acetaminophen or ibuprofen. However, she says, she does not recommend using these medications pro-actively. “Don’t take it to avoid side effects.”

What should people expect when it comes to their second dose?

Because people’s immune response depends on individual genetic differences, it’s hard to say whether side effects will be more or less severe after the second dose, Dr. Fritz says. But, he says, the common side effects don’t last more than a day or two.

For her part, Ms. Arnold says she’ll be prepared when it comes time to get her second dose. She plans to have Tylenol and water at her side, and will not schedule anything for the next day, so she can ride out any discomfort.

“I’m not going to lie, I’m a little scared,” she says. “I’m very eager to get it, though, because I want to continue with life.”

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