Most people who get a COVID or flu vaccine will experience at least one side effect, and in nearly all cases those effects are mild and short-lived. Injection-site pain, fatigue, headache, and muscle aches top the list for both vaccines, though COVID shots tend to provoke a noticeably stronger reaction than standard flu shots. The specifics vary by vaccine type, dose number, age, sex, and even whether you have had a previous infection, so the experience can feel very different from one person to the next.
The Most Common Side Effects
For mRNA COVID vaccines, the pattern that showed up consistently across large clinical trials is straightforward: injection-site pain is the single most reported reaction, followed by fatigue, muscle pain, headache, chills, joint pain, and fever. A systematic review of phase 1 through phase 3 trial data found that the majority of vaccine recipients reported at least one local or systemic side effect, but the effects were minor and resolved quickly.1PubMed Central. Safety and Adverse Events Related to COVID-19 mRNA Vaccines; a Systematic Review In a study of roughly 3,600 healthcare workers who received a second dose of the Pfizer vaccine, about three-quarters reported pain at the injection site, and over half reported at least one systemic reaction such as fatigue, muscle pain, or headache.2PubMed Central. Side effects among healthcare workers from a large Milan university hospital after second dose of BNT162b2 mRNA COVID-19 vaccine
Flu vaccines generally cause a lighter reaction. Most people notice soreness at the injection site and maybe a day of mild fatigue, and that is about it. In a head-to-head comparison of high-dose and standard-dose flu vaccines given to adults 65 and older, roughly a third of high-dose recipients and about a fifth of standard-dose recipients reported any local or systemic side effect, with severe reactions being rare in both groups.3PubMed Central. Comparison of Side Effects of the 2015–2016 High-Dose, Inactivated, Trivalent Influenza Vaccine and Standard Dose, Inactivated, Trivalent Influenza Vaccine in Adults ≥65 Years So if you have had a flu shot that barely bothered you and are wondering whether the COVID shot will be similar, expect somewhat more noticeable effects from the COVID vaccine, especially after a second dose or booster.
Why the Second Dose Hits Harder
A pattern people quickly noticed during the COVID vaccine rollout was that the second mRNA dose tended to cause stronger side effects than the first. This is not a sign that something went wrong. The systematic review data confirmed it: the incidence of systemic reactions increased after the second dose for both the Pfizer and Moderna vaccines.1PubMed Central. Safety and Adverse Events Related to COVID-19 mRNA Vaccines; a Systematic Review By the time you get that second shot, your immune system has already been primed by the first dose. It recognizes the target faster, mounts a bigger inflammatory response, and that amplified response is what you feel as fatigue, fever, and aches.
Research in animal models has helped pin down which immune signals are responsible. Cytokines like IL-6 and TNF-alpha, which are chemical messengers that coordinate inflammation, spike after mRNA vaccination and drive symptoms like fever and weight loss. After a booster, T cells ramp up production of TNF-alpha even further, which helps explain why subsequent doses can feel rougher.4Molecular Therapy. Mechanisms underlying systemic and local adverse reactions induced by mRNA-lipid nanoparticle vaccines The encouraging finding from that same research is that IL-6, one of the biggest contributors to feeling lousy, does not appear to play much of a role in building the protective immune response itself, which opens the door to future vaccines that might cause fewer side effects without sacrificing effectiveness.
Who Gets More Side Effects
Your age and sex are two of the biggest predictors of how you will feel after a COVID vaccine. Younger adults consistently report more side effects than older adults. A meta-analysis covering multiple vaccine types found that younger people had a higher incidence of both local and systemic reactions compared with older adults.5PubMed Central. The Impact of Age Difference on the Efficacy and Safety of COVID-19 Vaccines: A Systematic Review and Meta-Analysis That may sound counterintuitive, since older people are generally more vulnerable to disease, but it reflects the fact that younger immune systems tend to mount a more aggressive inflammatory response. An observational study in the UAE reached a similar conclusion and also flagged having a prior COVID infection and pre-existing health conditions as factors that increased the odds of post-vaccination side effects.6PubMed Central. Vaccine Side Effects Following COVID-19 Vaccination Among the Residents of the UAE-An Observational Study
Women tend to report more side effects than men, and this gap has shown up across multiple studies and countries. A study of medical staff in China found that women reported adverse reactions roughly twice as often as men after the first dose.7PubMed Central. Sex Differences in Adverse Reactions to an Inactivated SARS-CoV-2 Vaccine Among Medical Staff in China A study in Japan found women reported a median of five side effects after their second mRNA dose compared with two and a half for men, and women also took longer to recover.8PubMed Central. Impact of Sex and Age on mRNA COVID-19 Vaccine-Related Side Effects in Japan Research after booster doses found the same pattern, with women reporting a median of six symptoms compared with three for men.9Scientific Reports. Impact of sex and age on vaccine-related side effects and their progression after booster mRNA COVID-19 vaccine The reasons likely involve hormonal differences in immune regulation, but whatever the cause, it is real and reproducible. If you are a woman who felt worse than the men around you after a shot, it was not in your head.
Previous COVID Infection and Side Effects
If you had COVID before getting vaccinated, you were more likely to feel side effects afterward. A large prospective study tracking hundreds of thousands of vaccinated individuals in the UK found that people with a previous positive COVID test were substantially more likely to report systemic effects after a first Pfizer dose than people without known past infection. The same held for a first dose of the AstraZeneca vaccine and for the second Pfizer dose.10The Lancet Infectious Diseases. Vaccine side-effects and SARS-CoV-2 infection after vaccination in users of the COVID Symptom Study app in the UK: a prospective observational study The logic is similar to why second doses are rougher: a previously infected person’s immune system already has memory cells ready to react, so the vaccine triggers a faster, bigger response. This does not mean the vaccine is unsafe for people with prior infection. It simply means the body recognizes the target and fires up more quickly.
Getting Both Shots at Once
Many pharmacies now offer COVID and flu vaccines at the same visit, and people understandably wonder whether doubling up means doubling the side effects. A randomized trial studying co-administration of a Moderna booster with a seasonal flu vaccine found that pain at the injection site and muscle aches were the most common reactions in both groups, and the overall frequency, intensity, and duration of side effects were similar whether people received both vaccines at once or weeks apart.11PubMed Central. No immunological interference or concerns about safety when seasonal quadrivalent influenza vaccine is co-administered with a COVID-19 mRNA-1273 booster vaccine in adults: A randomized trial Getting them together did not compromise the immune response to either vaccine either. So if convenience matters to you and you want to get both out of the way in a single trip, the evidence supports that approach.
The Nocebo Effect Is Real
Here is something that surprises most people: a meaningful chunk of what gets reported as vaccine side effects appears to be driven by expectation rather than the vaccine itself. A meta-analysis of placebo arms across COVID vaccine trials found that after the first dose, nocebo responses accounted for roughly three-quarters of systemic side effects like headache and fatigue, and about a quarter of local effects like arm pain.12JAMA Network Open. Frequency of Adverse Events in the Placebo Arms of COVID-19 Vaccine Trials: A Systematic Review and Meta-analysis That does not mean people are faking it. These symptoms are genuinely felt. But if you go into a vaccine appointment convinced you will feel terrible, your brain may helpfully produce exactly that experience. A separate study found that vaccine hesitancy itself predicted a higher burden of nocebo side effects after COVID vaccination.13Scientific Reports. Vaccine hesitancy prospectively predicts nocebo side-effects following COVID-19 vaccination
This matters practically. If you are anxious about side effects, knowing that some of the symptoms are amplified by expectation can be genuinely helpful. It does not make the arm pain or the headache less real, but it reframes the experience: a mild ache that you might ignore in daily life gets interpreted as “the vaccine side effects starting” when you are primed to notice it. The after-the-second-dose dropoff in the nocebo proportion also makes sense: after the first dose, people get a more realistic picture of their own reaction, which recalibrates their expectations.
Managing Pain and Fever After Vaccination
Reaching for ibuprofen or acetaminophen after a vaccine is common, but there has been an ongoing question about whether pain relievers could blunt the immune response. The evidence is reassuring. A review of studies found that observational data consistently showed no effect of antipyretic use on antibody responses, while a handful of randomized trials found modest reductions in antibody levels of unclear real-world significance.14PubMed Central. Effect of antipyretic analgesics on immune responses to vaccination More directly relevant to COVID vaccines, a study of people who took an antipyretic before their Pfizer booster dose found no significant difference in antibody levels compared with those who did not.15Vaccine: X. No significant influence of pre-vaccination antipyretic use on specific antibody response to a BNT162b2 vaccine booster against COVID-19
An early study of the AstraZeneca vaccine actually permitted prophylactic acetaminophen before vaccination at some trial sites and found it significantly reduced pain, chills, muscle ache, and headache without affecting the immune response.16npj Vaccines. Use of analgesics/antipyretics in the management of symptoms associated with COVID-19 vaccination So if you are miserable after a shot, take something. The practical advice is simple: you do not need to suffer through it to “get the full benefit,” and there is no strong evidence that treating your symptoms undermines protection.
Do Worse Side Effects Mean Better Protection?
People who have a rough couple of days after vaccination often console themselves with “at least it means the vaccine is really working.” There is a kernel of truth here, but it is not a reliable rule. Research has found that a strong innate immune response after an mRNA vaccine was associated with higher odds of experiencing moderate symptoms.17PubMed Central. Innate immune response after BNT162b2 COVID-19 vaccination associates with reactogenicity And a four-year cohort study of hospital employees found that fever remained positively associated with post-vaccination antibody levels across several booster rounds.18PubMed. Longitudinal changes in the antibody responses and adverse reactions across successive SARS-CoV-2 mRNA vaccinations from the primary series to the JN.1-adapted booster in hospital employees But the relationship is loose, not absolute. Plenty of people who felt fine after their shot developed robust antibody responses. The absence of side effects does not mean the vaccine failed, and a miserable reaction is not a guarantee of superior immunity.
Protein-Based Versus mRNA Vaccines
Not everyone gets an mRNA vaccine, and the alternative formulations have a different side-effect profile. Novavax’s protein-based COVID vaccine (NVX-CoV2373) has been positioned as an option for people who want a non-mRNA choice. A comparative study found that within the first two days after vaccination, about 60% of Novavax recipients reported systemic symptoms compared with roughly 84% of mRNA vaccine recipients, and the average number of symptoms was lower as well.19PubMed Central. Reactogenicity Differences between Adjuvanted, Protein-Based and Messenger Ribonucleic Acid (mRNA)-Based COVID-19 Vaccines If you have found mRNA vaccines particularly unpleasant, the protein-based option is worth discussing with your pharmacist or doctor. For flu vaccines, adjuvanted versions designed for older adults have shown comparable tolerability to non-adjuvanted versions, though any adjuvanted vaccine can provoke slightly more local soreness.20PubMed. Immunogenicity and safety of MF59-adjuvanted quadrivalent influenza vaccine versus standard and alternate B strain MF59-adjuvanted trivalent influenza vaccines in older adults
Rare but Serious Reactions
The vast majority of vaccine side effects are nuisances, not dangers. But rare serious reactions do exist, and being aware of them matters, especially if they apply to your demographic. The most discussed is myocarditis, an inflammation of the heart muscle. A large systematic review found the highest risk after the second dose of an mRNA vaccine in young men: roughly 20 extra cases per 100,000 doses in men aged 18 to 24 receiving Moderna, and about 10 extra cases per 100,000 doses in boys aged 12 to 17 receiving Pfizer.21Epidemiologic Reviews. Age- and sex-stratified risks of myocarditis and pericarditis attributable to COVID-19 vaccination These cases were overwhelmingly mild and self-resolving, but chest pain, shortness of breath, or a pounding heart within a week or so of an mRNA vaccine warrants medical attention.
Important context: a UK-based study using national health records found that the risk of myocarditis after a COVID infection itself was several times higher than after vaccination. In men younger than 40, a second dose of the Moderna vaccine did carry a higher rate of excess myocarditis events than a positive COVID test, but for other vaccines and age groups, the infection posed the greater risk.22PubMed Central. Risk of Myocarditis After Sequential Doses of COVID-19 Vaccine and SARS-CoV-2 Infection by Age and Sex
Guillain-Barré syndrome, a nerve disorder that can cause muscle weakness, was flagged early on as a concern with viral vector vaccines (like AstraZeneca and Johnson & Johnson) rather than mRNA vaccines. A prospective surveillance study found the incidence of GBS after viral vector vaccines was about 4.5 per million doses, compared with less than 1 per million doses after mRNA vaccines.23PubMed Central. Real-world data on the incidence and risk of Guillain–Barré syndrome following SARS-CoV-2 vaccination: a prospective surveillance study Viral vector COVID vaccines have been largely phased out in most countries, in part because of these safety signals, so this is less relevant for current vaccination decisions.
Severe allergic reactions, including anaphylaxis, remain extremely rare. The concern has centered on polyethylene glycol (PEG), an ingredient in mRNA vaccines that has been identified as a possible trigger.24PubMed Central. Hypersensitivity to polyethylene glycol in adults and children: An emerging challenge In practice, even people with confirmed PEG allergy have tolerated COVID vaccination when managed carefully. A clinical protocol that screened over 400 patients at higher risk for vaccine allergy identified a small number with true PEG sensitivity, and all six PEG-allergic patients who went on to be vaccinated tolerated it without a reaction.25PubMed Central. Experience with polyethylene glycol allergy-guided risk management for COVID-19 vaccine anaphylaxis This is why vaccination sites have you wait 15 to 30 minutes afterward — the observation period exists to catch the vanishingly rare anaphylactic reaction when it can be treated immediately.
Menstrual Changes
Many women reported changes to their periods after COVID vaccination, and for a while this was dismissed or underinvestigated. The evidence now supports these reports. A systematic review found that women who received COVID vaccines experienced menstrual abnormalities including heavier bleeding, irregular timing, and changes in cycle length, though these were typically intermittent and resolved on their own.26PubMed Central. Menstrual abnormalities after COVID-19 vaccines: A systematic review A study from Turkey confirmed that menstrual delay, heavier bleeding, longer duration, and early onset were all more common after vaccination compared to before.27PubMed Central. Examining the effect of COVID-19 vaccines on the menstrual cycle: A study from Turkey If you notice a cycle or two that seem off after a vaccine, this is a recognized temporary effect. It does not indicate any lasting harm to fertility or reproductive health.
Delayed Skin Reactions
Some people develop a red, itchy, sometimes swollen patch near the injection site days after vaccination, well past the window when you would expect normal soreness. This was informally called “COVID arm” and was documented more often with the Moderna vaccine. A case series found these delayed localized reactions appeared a median of seven days after the first dose, presenting as pink, sometimes ring-shaped plaques. Biopsies showed a pattern consistent with a delayed-type hypersensitivity reaction driven by the immune system rather than an infection or allergic emergency.28JAMA Dermatology. Delayed Localized Hypersensitivity Reactions to the Moderna COVID-19 Vaccine: A Case Series These reactions look alarming but are not dangerous, and they do not mean you should skip your second dose. They typically resolve on their own or with over-the-counter antihistamines.
Vaccination During Pregnancy
Pregnant women were initially excluded from vaccine trials, which left a frustrating gap in the evidence. Since then, observational data have filled in a clearer picture. A systematic review examining outcomes in pregnant women who received Pfizer or Moderna vaccines found that rates of adverse pregnancy outcomes were similar between vaccinated and unvaccinated women. In one matched cohort, rates of postpartum hemorrhage, small-for-gestational-age babies, intensive care admissions, fetal abnormalities, and stillbirth were comparable in both groups.29PubMed Central. Complications of COVID-19 Vaccines during Pregnancy; a Systematic Review The most commonly reported side effects in pregnant vaccine recipients were the same ones everyone else gets: injection-site pain and the usual systemic complaints. Serious reactions like uterine contractions and vaginal bleeding were reported in a small fraction of cases. The general guidance from major health agencies remains that COVID vaccination during pregnancy is recommended, given the well-documented risks of COVID infection in pregnant women.
How Side Effects Get Tracked at Scale
The COVID vaccine rollout featured the largest vaccine safety monitoring effort in history. In the United States, the CDC’s v-safe program allowed vaccinated individuals to report their symptoms through a smartphone-based health checker. Over 9 million people submitted more than 131 million surveys through the system, providing near real-time data on how vaccines were performing outside the controlled environment of clinical trials.30PubMed Central. The v-safe after vaccination health checker: Active vaccine safety monitoring during CDC’s COVID-19 pandemic response This kind of active surveillance, where people are prompted to report rather than waiting for them to file a complaint, captures a much more complete picture of common reactions than traditional passive reporting systems. It is one reason the data on side effects from COVID vaccines is substantially richer than what we have for most other vaccines, including seasonal flu shots, and it helped identify patterns like the sex and age differences discussed above relatively quickly.