Flu and COVID Vaccine Side Effects: What to Expect and When

Vaccines for the flu and COVID-19 help reduce illness severity, hospitalizations, and complications. While generally safe, they can cause side effects as the immune system responds. These reactions range from mild, short-lived symptoms to rare but more serious effects.

Common Postvaccination Effects

Many individuals experience temporary symptoms after flu or COVID-19 vaccination, signaling the body’s immune response. The most common reaction is localized pain at the injection site, often with redness or swelling. This occurs due to inflammation triggered by the vaccine components. A study in The Lancet Infectious Diseases found that up to 70% of mRNA COVID-19 vaccine recipients reported mild to moderate arm discomfort, usually resolving within 48 hours. Flu vaccines, particularly inactivated virus formulations, cause similar soreness in about 30% of recipients.

Beyond injection-site reactions, systemic symptoms like fatigue, headache, and muscle aches are common. These typically appear within a day and last one to three days. A JAMA review of data from over 6 million individuals found that fatigue was reported by about 50% of mRNA COVID-19 vaccine recipients and 20% of flu vaccine recipients. Fever and chills, though less frequent, are more common with COVID-19 vaccines. Clinical trials for Pfizer-BioNTech and Moderna vaccines showed that around 15% of participants developed mild fever, especially after the second dose. Flu vaccines, particularly live attenuated versions, have been linked to low-grade fever in about 10% of recipients, primarily younger individuals.

Gastrointestinal symptoms, though less discussed, can also occur. Nausea has been reported in about 10% of COVID-19 vaccine recipients, according to CDC data. Flu vaccines, especially intranasal versions, have been associated with mild digestive issues like transient nausea or appetite loss. These effects are generally short-lived and require no medical intervention.

Less Common Immune Responses

While most side effects are mild, some individuals experience less common reactions, including allergic responses, neurological symptoms, and other systemic effects. Recognizing these helps determine when medical attention may be needed.

Allergic Reactions

Severe allergic reactions to flu and COVID-19 vaccines are rare. Anaphylaxis, a rapid-onset allergic response, occurs in about 2 to 5 cases per million mRNA COVID-19 vaccine doses, according to the CDC. Symptoms, which appear within minutes, include breathing difficulty, facial or throat swelling, and a sudden drop in blood pressure. The primary triggers are polyethylene glycol (PEG) in mRNA vaccines and polysorbate in some flu vaccines. Flu vaccines made using egg-based methods may pose a risk for individuals with severe egg allergies, though most can receive them safely under medical supervision. Vaccination sites are equipped with emergency treatments like epinephrine, and individuals with a history of severe allergies should remain under observation for at least 15 to 30 minutes postvaccination.

Neurological Manifestations

Some individuals report neurological symptoms after flu or COVID-19 vaccination, though these are uncommon. Guillain-Barré syndrome (GBS), a rare autoimmune disorder affecting the peripheral nervous system, has been linked to certain flu vaccines at a rate of 1 to 2 cases per million doses, according to a 2021 Neurology study. It typically develops within weeks and presents with progressive muscle weakness and tingling. While the exact cause remains unclear, it is believed that, in rare cases, the immune response may mistakenly target nerve cells. COVID-19 vaccines have been associated with transient neurological effects such as dizziness, tingling, or headaches, though no confirmed link to GBS exists. The CDC and FDA continue monitoring vaccine safety through surveillance systems like VAERS.

Other Potential Symptoms

Some individuals experience additional symptoms, such as joint pain, reported by about 5% of mRNA COVID-19 vaccine recipients, according to The New England Journal of Medicine. This discomfort, often in the knees or hands, is typically mild and resolves within days. Skin reactions, including delayed-onset rashes or swelling at the injection site, have also been noted. A study in JAMA Dermatology described cases of “COVID arm,” where redness and itching appeared days after vaccination but resolved without treatment. Flu vaccines have been linked to temporary lymph node swelling, particularly in the neck or armpit, as the immune system responds to the vaccine. While usually harmless, persistent or worsening symptoms should be discussed with a healthcare provider.

Timing of Symptom Appearance

The onset of symptoms depends on the vaccine type and individual response. Most effects appear within the first 48 hours. Localized reactions, such as arm soreness or mild swelling, often develop within hours and peak within 24 hours before subsiding.

Systemic symptoms like fatigue, headache, and muscle aches typically emerge within 12 to 24 hours and last one to three days. Fever, when present, usually appears within the first day and rarely lasts beyond 48 hours. A Journal of Infectious Diseases review found that fever was most common after the second dose of mRNA COVID-19 vaccines, resolving within two days. Flu vaccines, particularly live attenuated versions, may also cause mild fever, especially in younger individuals.

Some reactions appear later. A subset of COVID-19 vaccine recipients have reported “delayed injection-site reaction,” with redness and swelling developing four to seven days postvaccination. A study in The New England Journal of Medicine found that this reaction resolved without treatment. Mild lymph node swelling, particularly in the armpit or neck, has been observed within the first week and can last up to 10 days.

Factors Influencing Reaction Intensity

The severity of side effects varies based on age, prior vaccination, and health status. Individuals previously vaccinated against flu or COVID-19 may experience milder symptoms due to an adaptive immune response. Those receiving a vaccine for the first time or after a long gap may report stronger reactions. A study in Vaccine found that individuals with prior COVID-19 infection often experience heightened side effects, particularly after the first dose, likely due to pre-existing immune priming.

Dosage and formulation also impact symptom intensity. mRNA COVID-19 vaccines, such as Pfizer-BioNTech and Moderna, tend to cause stronger systemic effects than protein-based or inactivated flu vaccines due to their higher immunogenicity. Booster doses can elicit varying reactions depending on the time since prior immunization. Studies show that shorter intervals between doses may lead to more transient fatigue and injection-site discomfort, while longer gaps can sometimes result in a stronger response.

Coadministration Considerations

Receiving flu and COVID-19 vaccines together is generally safe and does not reduce efficacy. A study in The Lancet Respiratory Medicine found that individuals receiving both vaccines at the same appointment developed similar immune responses compared to those vaccinated separately. Coadministration is particularly beneficial for high-risk individuals, such as older adults and those with underlying conditions, ensuring timely protection against both viruses.

Some experience slightly higher rates of mild systemic reactions, such as fatigue and muscle aches, when receiving both vaccines simultaneously. This likely results from the immune system responding to multiple antigens at once. However, these effects are short-lived, typically resolving within days. Some may choose to space out vaccinations for comfort, though there is no clinical necessity to do so. The decision should be based on personal preference, health status, and vaccine availability.

Age-Related Patterns

Side effect intensity varies by age. Younger adults and adolescents tend to experience stronger systemic reactions, such as fever, chills, and muscle aches, particularly after mRNA COVID-19 vaccines. This is due to a more robust immune response. Clinical trials for Pfizer-BioNTech and Moderna vaccines showed that individuals under 30 were more likely to report fatigue and fever than older recipients. Flu vaccines follow a similar trend, with younger individuals more frequently experiencing mild fever and muscle soreness, especially after the live attenuated intranasal formulation.

Older adults generally report fewer side effects due to immunosenescence, the gradual decline in immune function. While this results in a milder postvaccination experience, it can also mean a slightly lower antibody response, which is why high-dose flu vaccines are recommended for individuals over 65. These formulations provide stronger immune protection without significantly increasing side effect severity. For COVID-19 vaccines, older adults have shown strong protection against severe disease despite reporting fewer transient symptoms. They may be more prone to delayed-onset effects, such as mild lymph node swelling, but these occurrences remain rare and self-limiting.