If the COVID vaccine made you feel terrible, it probably offered you better protection, new study suggests

Feeling sick after getting a COVID shot could actually be a good sign, according to a new scientific study that has found a link between post-vaccination side effects and a stronger immune response.

In operation published on Fridayresearchers from Columbia University, the University of Vermont and Boston University said side effects such as fatigue and high fever could be signs that the vaccine was effective.

Using survey data and blood samples from 928 American adults, the research team analyzed the relationship between self-reported post-vaccination symptoms and antibody responses. The average age of the sampled group was 65 years.

Dried blood spot samples were collected from participants in February 2021, the same month that subjects were invited to fill out questionnaires about their history of vaccination against COVID. Everyone participating must have had their last vaccination at least two weeks prior to submitting blood samples.

All study participants were vaccinated with two doses of Pfizer-BioNTech or Modern The COVID vaccine, both of which use mRNA technology to stimulate an immune response.

Nearly half of the participants reported experiencing “systemic symptoms” — such as fever, chills, nausea, vomiting, headache or fatigue — after any dose of the vaccine. A further 12% said they had local symptoms such as hand inflammation or a rash near the injection site, and 40% said they had no side effects at all.

The researchers found that post-vaccination symptoms were associated with younger women, previous COVID infection and the Moderna vaccine.

The scientists also measured the level of antibodies in the blood that work by targeting the spike protein of COVID-19.

Immune reactions to the vaccines were observed in almost all participants, but higher antibody reactivity was observed in those who reported local or systemic symptoms after vaccination. Among those who experienced side effects after vaccination, 99% were found to have an antibody response to immunization, compared to 98% of the asymptomatic group.

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The authors of the paper noted that almost all participants showed a positive antibody response to the vaccines, but said that “systemic symptoms nevertheless remained associated with a greater antibody response in multivariable-adjusted models.”

They also acknowledged that their older and predominantly white sample limited the impact of their study, saying more research is needed.

However, they argued that their findings support “the reshaping of post-vaccination symptoms as a signal of vaccine efficacy.”

Side effects

Although side effects from the COVID-19 vaccine are quite common, they vary from person to person. According to the US Centers for Disease Control and Prevention (CDC), most common side effects after the second injection or booster, they are mild and include fever, headache, fatigue, and pain at the injection site.

The two newly approved bivalent vaccines—manufactured by Pfizer-BioNTech and Moderna to specifically target the original strain of COVID and the rapidly spreading Omicron subvariants—caused similar side effects to the original vaccines during human testing.

Although side effects of vaccines exist a sign that the immunization is workingthe doctors said that there are no symptoms after the vaccination does not mean there was no immune response, and there is there is no major scientific evidence that those with more obvious side effects are better protected.

Last year, study on the potential link between adverse effects of the COVID vaccine and the immune response found that a lack of symptoms after vaccination “does not equal a lack of vaccine-induced antibodies.”

This story was originally published on Fortune.com

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