Why the side effects are stronger in the second dose of the covid-19 vaccine

It is common to experience mild side effects after getting the coronavirus vaccine, but people who get the two-dose vaccine tend to have stronger side effects after the second shot.

The effects can be headache, fatigue, body aches, fever, and nausea. They are generally mild and go away on their own within a few days, according to federal health authorities.

But why should the side effects be more intense after the second dose?

Medical experts say it has to do with how the immune system reacts to the virus.

The Pfizer-BioNTech and Moderna two-dose vaccines are of the mRNA type, which contain a genetic code that teaches the body to make a protein that activates an immune response and helps produce antibodies.

When a person who has never been exposed to the coronavirus is vaccinated, their immune system recognizes the protein as an external agent and triggers a reaction to it, but not very strong, explains Dr. Beth Kassanoff, an internal medicine specialist at North Texas Preferred Health. Partners and president of the Dallas County Medical Society.

With the second dose of the vaccine, the person’s immune system “remembers” the protein and reacts stronger.

“When you get the second injection, the immune reaction is much stronger because the body already has some antibodies,” he said. “They are ready to fight the infection.”

Although that response can cause flu-like symptoms, it is actually a good sign and means the vaccine is doing its job.

But at the same time, people with no side effects have nothing to worry about, Kassanoff added. Just because you are not experiencing side effects does not mean that the body is not reacting to the injection.

“I have seen people of all ages, some with a lot of pain, a little fever, fatigue …”, he said.

“But one of my colleagues, 50, had absolutely no effect with the second one. In fact, he was a bit worried about it at first, but he did an antibody test and responded quite well. It was very interesting, because you can’t predict it ”.

Health experts are still learning why some people experience more side effects than others, but they say it has to do with age and exposure to the virus.

“One pattern we’ve been seeing is that older people have fewer side effects than younger people, but we don’t know why,” said Katelyn Jetelina, an epidemiologist at UTHealth School of Public Health in Dallas.

“Another thing we’ve been seeing is that people who have had COVID before and then get vaccinated have stronger side effects. And it makes sense, because that means that they already have a certain degree of immunity and that the body recognizes the antibodies in that vaccine and is reacting ”.

Kassanoff says he has heard of cases of people who have had covid symptoms for a long time after they no longer test positive for the virus, and their symptoms more or less resolve after they get vaccinated.

But scientists say it is not yet clear why this is the case.

“I have cared for entire families, with parents and adult children, and everyone has a different reaction,” Kassanoff said.

“Even within the same family, and even roughly the same age, with roughly the same genetics, sometimes people have a stronger reaction.”

That behavior is not unique to coronavirus vaccines. With other vaccines too, some people react differently than others, he said.

As more people get vaccinated, more will be known about the side effects, experts say.

Although the side effects of the second dose can be scary, that shouldn’t discourage people from getting vaccinated, says Jetelina.

“It’s hard, let’s say, to sign up to get sick,” he said.

“Not everyone gets sick and, honestly, most people don’t get sick; We only hear about those terrible cases or people who get sick. What I would say is hold on, take Tylenol and rest all day, and it goes away. I assure you that it is removed ”.

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