Key TakeawaysResearch shows that females may have a stronger reaction to the flu shot than males do, as well as have more side effects.Sex differences in immune function are likely a factor, as is behavior; females are more likely to seek medical attention for symptoms than men.Experts said the difference is not significant enough to be worrisome, and that everyone should get the flu shot if they’re able to.Women rolling up their sleeves to get theirannual flu shotmight have a stronger reaction to the jab than men do, according to a new study.Researchers from the University of Montreal analyzed data from 18 studies on flu shot reactions in more than 34,000 participants. The study showed that within seven days of having the vaccine, female participants were more likely to have a reaction at the injection site as well as systemic reactions like headache, muscle aches, and fever.The researchers also looked at the risks for different kinds of reactions:The risk for injection site reactions was 29% higher for younger females (aged 18–64) and 43% higher for older females (aged 65+) compared to males.For systemic reactions, the risk was 25% higher for younger females, and 27% higher for older females.For severe reactions—for example, a high fever that prevented their daily activities—the risk was about twice as high in females as in males.Current Flu Case Numbers in the U.S.Marilou Kiely, PhD, who did the research as a fellow at the University of Montreal, told Verywell that even though there is a difference in vaccine response between the sexes, it’s not really big enough to be concerning. According to the study, the risk was only 115 extra cases out of 1,000 vaccines for injection site reactions and 74 extra cases out of 1,000 for systemic reactions.Females might have more side effects from a flu shot but could also have more protection from it.“The higher response following vaccination could be associated with better protection,” Kiely said.Why Would Females React More to a Flu Shot?William Schaffner, MD, a professor of infectious diseases at the Vanderbilt University School of Medicine, told Verywell that there could be a few reasons why females respond differently to vaccines than males.One is the social, cultural, and behavioral conditioning of women compared to men. Some research has suggested that women are more likely to seek medical attention for health symptoms,so they might be more likely to pay attention to vaccine side effects—a factor that the Canadian researchers also pointed out.While the biological basis wasn’t revealed in the study, Schaffner said that “there could be biological reasons for this difference in reaction as there are lots of differences between males and females.”For example, estrogen levels, which are higher in females, may have something to do with the heightened response. Or perhaps it’s something about male biological factors. For example, Schaffner wondered if testosterone suppresses the local responses to a vaccine.A study from Harvard also showed that sex may influence immune responses. The researchers found that generally, females “induce stronger immune functions and higher antibody levels, composed of more functional antibodies, but also experience more adverse reactions to vaccination.”Sex differences have been seen with other vaccines, too. Females have reported having stronger reactions toCOVID-19 vaccines, which may also be attributed to more reactive immune systems.Most Common Flu Shot Side EffectsWhat to Do for Flu Shot Side EffectsBoth Kiely and Schaffner said that healthcare providers should tell female patients that they could have a more intense response to a vaccine. This warning allows women to be informed and prepared about the possible side effects.“If people are provided information about anticipated local reactions, they’re less likely to get as concerned about them,” said Schaffner.Kiely added that if patients understand how sex differences can influence reaction, they’re less likely to have vaccine hesitancy.“We know that concerns about safety can be associated with vaccine hesitancy,” she said. “We think that transparentcommunication of risksis a better way to deal with hesitancy, and that communication could increase trust in vaccines in the long run.”If female vaccine recipients do have amild reaction after their vaccine, Schaffner said they can take over-the-counter (OTC) pain relievers/fever reducers, such as Tylenol. However, it’s best to take these meds after you get a shot, not before.“There is some really weak data to suggest that if you take medication such as Tylenol before you get the immunization, that [medication] may actually impair the immune response,” said Schaffner. “The data is not that strong, but it has led the CDC to distinguish between taking Tylenol before and taking Tylenol after, and they recommend taking Tylenol after because you don’t want to diminish your protection against flu or any other illness for which you take a vaccine.”What This Means For YouFemales might respond more strongly to vaccines than males, which can make them more likely to have side effects. Researchers think there are lots of social and biological factors that could contribute to sex differences in vaccine reactions, but the differences aren’t big enough that people should be worried about them. If you have mild side effects after you get a vaccine, taking an OTC pain reliever can help.

Key TakeawaysResearch shows that females may have a stronger reaction to the flu shot than males do, as well as have more side effects.Sex differences in immune function are likely a factor, as is behavior; females are more likely to seek medical attention for symptoms than men.Experts said the difference is not significant enough to be worrisome, and that everyone should get the flu shot if they’re able to.

Key Takeaways

Research shows that females may have a stronger reaction to the flu shot than males do, as well as have more side effects.Sex differences in immune function are likely a factor, as is behavior; females are more likely to seek medical attention for symptoms than men.Experts said the difference is not significant enough to be worrisome, and that everyone should get the flu shot if they’re able to.

Women rolling up their sleeves to get theirannual flu shotmight have a stronger reaction to the jab than men do, according to a new study.

Researchers from the University of Montreal analyzed data from 18 studies on flu shot reactions in more than 34,000 participants. The study showed that within seven days of having the vaccine, female participants were more likely to have a reaction at the injection site as well as systemic reactions like headache, muscle aches, and fever.

The researchers also looked at the risks for different kinds of reactions:

Current Flu Case Numbers in the U.S.

Marilou Kiely, PhD, who did the research as a fellow at the University of Montreal, told Verywell that even though there is a difference in vaccine response between the sexes, it’s not really big enough to be concerning. According to the study, the risk was only 115 extra cases out of 1,000 vaccines for injection site reactions and 74 extra cases out of 1,000 for systemic reactions.

Females might have more side effects from a flu shot but could also have more protection from it.

“The higher response following vaccination could be associated with better protection,” Kiely said.

Why Would Females React More to a Flu Shot?

William Schaffner, MD, a professor of infectious diseases at the Vanderbilt University School of Medicine, told Verywell that there could be a few reasons why females respond differently to vaccines than males.

One is the social, cultural, and behavioral conditioning of women compared to men. Some research has suggested that women are more likely to seek medical attention for health symptoms,so they might be more likely to pay attention to vaccine side effects—a factor that the Canadian researchers also pointed out.

While the biological basis wasn’t revealed in the study, Schaffner said that “there could be biological reasons for this difference in reaction as there are lots of differences between males and females.”

For example, estrogen levels, which are higher in females, may have something to do with the heightened response. Or perhaps it’s something about male biological factors. For example, Schaffner wondered if testosterone suppresses the local responses to a vaccine.

A study from Harvard also showed that sex may influence immune responses. The researchers found that generally, females “induce stronger immune functions and higher antibody levels, composed of more functional antibodies, but also experience more adverse reactions to vaccination.”

Sex differences have been seen with other vaccines, too. Females have reported having stronger reactions toCOVID-19 vaccines, which may also be attributed to more reactive immune systems.

Most Common Flu Shot Side Effects

What to Do for Flu Shot Side Effects

Both Kiely and Schaffner said that healthcare providers should tell female patients that they could have a more intense response to a vaccine. This warning allows women to be informed and prepared about the possible side effects.

“If people are provided information about anticipated local reactions, they’re less likely to get as concerned about them,” said Schaffner.

Kiely added that if patients understand how sex differences can influence reaction, they’re less likely to have vaccine hesitancy.

“We know that concerns about safety can be associated with vaccine hesitancy,” she said. “We think that transparentcommunication of risksis a better way to deal with hesitancy, and that communication could increase trust in vaccines in the long run.”

If female vaccine recipients do have amild reaction after their vaccine, Schaffner said they can take over-the-counter (OTC) pain relievers/fever reducers, such as Tylenol. However, it’s best to take these meds after you get a shot, not before.

“There is some really weak data to suggest that if you take medication such as Tylenol before you get the immunization, that [medication] may actually impair the immune response,” said Schaffner. “The data is not that strong, but it has led the CDC to distinguish between taking Tylenol before and taking Tylenol after, and they recommend taking Tylenol after because you don’t want to diminish your protection against flu or any other illness for which you take a vaccine.”

What This Means For YouFemales might respond more strongly to vaccines than males, which can make them more likely to have side effects. Researchers think there are lots of social and biological factors that could contribute to sex differences in vaccine reactions, but the differences aren’t big enough that people should be worried about them. If you have mild side effects after you get a vaccine, taking an OTC pain reliever can help.

What This Means For You

Females might respond more strongly to vaccines than males, which can make them more likely to have side effects. Researchers think there are lots of social and biological factors that could contribute to sex differences in vaccine reactions, but the differences aren’t big enough that people should be worried about them. If you have mild side effects after you get a vaccine, taking an OTC pain reliever can help.

4 SourcesVerywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read oureditorial processto learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.Kiely M, Tadount F, Lo E, et al.Sex differences in adverse events following seasonal influenza vaccines: a meta-analysis of randomised controlled trials.J Epidemiol Community Health. Published online September 21, 2023. doi:10.1136/jech-2023-220781Fink AL, Klein SL.Sex and gender impact immune responses to vaccines among the elderly.Physiology (Bethesda). 2015;30(6):408-416. doi:10.1152/physiol.00035.2015The Conversation.Why women see their GP more than men.Fischinger S, Boudreau CM, Butler AL, Streeck H, Alter G.Sex differences in vaccine-induced humoral immunity.Semin Immunopathol. 2019;41(2):239-249. doi:10.1007/s00281-018-0726-5

4 Sources

Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read oureditorial processto learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.Kiely M, Tadount F, Lo E, et al.Sex differences in adverse events following seasonal influenza vaccines: a meta-analysis of randomised controlled trials.J Epidemiol Community Health. Published online September 21, 2023. doi:10.1136/jech-2023-220781Fink AL, Klein SL.Sex and gender impact immune responses to vaccines among the elderly.Physiology (Bethesda). 2015;30(6):408-416. doi:10.1152/physiol.00035.2015The Conversation.Why women see their GP more than men.Fischinger S, Boudreau CM, Butler AL, Streeck H, Alter G.Sex differences in vaccine-induced humoral immunity.Semin Immunopathol. 2019;41(2):239-249. doi:10.1007/s00281-018-0726-5

Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read oureditorial processto learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.

Kiely M, Tadount F, Lo E, et al.Sex differences in adverse events following seasonal influenza vaccines: a meta-analysis of randomised controlled trials.J Epidemiol Community Health. Published online September 21, 2023. doi:10.1136/jech-2023-220781Fink AL, Klein SL.Sex and gender impact immune responses to vaccines among the elderly.Physiology (Bethesda). 2015;30(6):408-416. doi:10.1152/physiol.00035.2015The Conversation.Why women see their GP more than men.Fischinger S, Boudreau CM, Butler AL, Streeck H, Alter G.Sex differences in vaccine-induced humoral immunity.Semin Immunopathol. 2019;41(2):239-249. doi:10.1007/s00281-018-0726-5

Kiely M, Tadount F, Lo E, et al.Sex differences in adverse events following seasonal influenza vaccines: a meta-analysis of randomised controlled trials.J Epidemiol Community Health. Published online September 21, 2023. doi:10.1136/jech-2023-220781

Fink AL, Klein SL.Sex and gender impact immune responses to vaccines among the elderly.Physiology (Bethesda). 2015;30(6):408-416. doi:10.1152/physiol.00035.2015

The Conversation.Why women see their GP more than men.

Fischinger S, Boudreau CM, Butler AL, Streeck H, Alter G.Sex differences in vaccine-induced humoral immunity.Semin Immunopathol. 2019;41(2):239-249. doi:10.1007/s00281-018-0726-5

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