What Are the Side Effects of the COVID-19 Vaccine?

Most people who receive a COVID-19 vaccine experience mild, short-lived side effects that resolve within a day or two. Injection-site pain, fatigue, headache, muscle aches, and low-grade fever are by far the most common reactions, and they reflect the immune system doing exactly what the vaccine was designed to trigger. Serious side effects do exist but are rare, and the picture varies meaningfully depending on which vaccine you received, how many doses you have had, your age, your sex, and whether you had a prior COVID-19 infection.

The Everyday Side Effects Most People Get

The side effects that show up in the first day or two after a COVID-19 shot are remarkably consistent across studies and countries. In a large Pakistani survey of 1,000 Pfizer recipients, about 70% reported burning or pain at the injection site and fever after the first dose, while roughly 63% reported muscle pain. Joint pain, sore throat, swelling at the injection site, and chills followed close behind, each affecting roughly half to 60% of participants.¹ Studies in Poland found that over 90% of people reported at least one reaction after a first dose of either Pfizer or AstraZeneca, though the types and intensity varied by vaccine brand.²

These reactions are not signs that something has gone wrong. They reflect the transient production of signaling molecules called type I interferons, part of the normal immune response to what the body perceives as a foreign invader.³ The soreness, fatigue, and low-grade fever are essentially collateral effects of your immune system ramping up. They typically peak within 24 hours and fade by the second or third day.

Why the Second Dose Often Feels Worse

If you felt fine after your first shot but were floored after the second, you are not imagining things. In the Polish survey data, about 55% of Pfizer recipients said adverse reactions were worse after dose two, while only about 16% said they were milder.² The Pakistani data told a similar story, with rashes jumping to about 61% after the second Pfizer dose and lymph node swelling becoming more prominent.¹ A study of adolescents in Saudi Arabia likewise found side effects more common after dose two.⁴

The reason is straightforward: by the time the second dose arrives, your immune system has already been primed by the first. It recognizes the spike protein faster and mounts a stronger response, which means more inflammation and more of the symptoms that go along with it. This is a feature of any multi-dose vaccine, not something unique to COVID-19 shots.

Who Gets Hit Harder

Not everyone reacts the same way, and several patterns have emerged across large studies.

Sex Differences

Women consistently report more side effects than men. A retrospective study found that about 77% of female recipients experienced adverse effects compared with 65% of males. Women reported more severe injection-site pain, more skin reactions like swelling and rash, more systemic symptoms like fatigue and nausea, and more cardiopulmonary complaints.⁵ This pattern holds up in adolescent populations too, with more female participants reporting side effects than males in a Saudi cohort of 12- to 18-year-olds.⁴ The reasons likely involve hormonal differences in immune regulation; estrogen tends to amplify inflammatory responses, which is also why women develop stronger antibody responses to many vaccines but pay for it in reactogenicity.

Age

Younger adults tend to have stronger reactions than older adults. A UAE observational study found that people under 55 had higher odds of developing side effects compared with older recipients.⁶ Among very old, community-dwelling adults, there were significant differences by age group, with mild reactions more common in the 75-to-84 bracket than in those 85 and older.⁷ This tracks with the general decline of immune responsiveness with age.

Prior COVID-19 Infection

If you had COVID-19 before getting vaccinated, expect a rougher ride. A large UK prospective study using the COVID Symptom Study app found that people with a previous positive test were roughly two to four times more likely to report systemic effects after vaccination than those without known past infection, depending on the vaccine and dose.⁸ The UAE study confirmed this, finding that a history of COVID-19 infection was independently associated with higher odds of post-vaccination adverse effects.⁶

Children and Infants

Children’s side effect profiles are generally milder than adults’. In a large study of young children receiving either the Pfizer or Moderna pediatric vaccines, reactions were reported in about 47% of kids, with irritability or fussiness being the most common complaint at about 30%, followed by local injection-site reactions at roughly 21% and fever at about 14%. Severe reactions were reported for fewer than 1% of children who had any reaction, and about 1.5% were seen by a healthcare professional for a reaction. No deaths were reported.⁹

Among older children aged 12 to 18, about 60% reported at least one side effect in a Saudi study, with injection-site pain leading the list at 90% of those who reported anything, followed by fatigue and fever.⁴ The overall picture for children is reassuring: side effects are common in the sense that many kids experience mild discomfort, but truly serious events remain rare.

Pregnancy and Breastfeeding

Pregnant women showed a trend toward fewer systemic effects after the first dose in a Mexican survey, though the finding was borderline.¹⁰ Lactating women experienced fewer local symptoms after the first dose but more after the second. Among breastfed infants whose mothers were vaccinated, irritability was the most reported symptom, but the severity was similar across groups.¹⁰

A review of safety data in lactating women found no serious side effects in mothers or infants across numerous large studies. Although trace amounts of vaccine mRNA can be transiently detected in breast milk, there is no evidence it survives an infant’s digestive tract or causes any harm to breastfeeding babies.¹¹

Myocarditis and Heart Inflammation

The most widely discussed rare side effect of mRNA COVID-19 vaccines is myocarditis, an inflammation of the heart muscle. It occurs most often in younger males, typically within the first few days after the second dose. Symptoms include chest pain (present in 95-100% of identified cases), shortness of breath, palpitations, and fatigue.¹² The elevated risk is concentrated in males aged 16 to 24, with the Moderna vaccine carrying a somewhat higher rate than Pfizer in this group.¹³

The critical context, though, is how rare this is and how it compares to the alternative. A Spanish hospital study found the incidence of myocarditis or pericarditis in people who received at least one mRNA dose was about 5 per 100,000, compared with 70 per 100,000 in unvaccinated individuals and 200 per 100,000 in people with COVID-19 infection. Cases linked to COVID-19 infection were also more severe and had higher mortality.¹⁴ A large UK study similarly showed that myocarditis risk after a positive SARS-CoV-2 test was several times higher than after any vaccine dose.¹⁵ And among adolescents and young adults specifically, a cohort study of four million people found that vaccination was associated with 85% lower myocarditis risk compared with infection.¹⁶

When vaccine-related myocarditis does occur, it generally follows a mild course. Most patients recover fully with short hospital stays, preserved heart function, and no lasting complications. ICU admissions are rare, and systematic reviews have reported no deaths among vaccinated individuals who developed myocarditis.¹³

Blood Clots With Adenoviral Vector Vaccines

A separate rare complication called vaccine-induced immune thrombotic thrombocytopenia, or VITT, was identified in 2021 in connection with adenoviral vector vaccines like AstraZeneca’s and Johnson & Johnson’s. VITT involves unusual blood clots paired with low platelet counts, driven by the formation of antibodies that activate platelets in a mechanism resembling heparin-induced thrombocytopenia.¹⁷ The antibodies involved target a protein on platelets called PF4, and these have been identified in almost all confirmed patients.¹⁸ Additional research has pointed to aberrant splicing of the adenoviral vector DNA encoding the spike protein as a possible contributing mechanism.¹⁹

VITT was a major reason several countries restricted or discontinued the use of adenoviral vector COVID-19 vaccines in favor of mRNA alternatives. It does not occur with mRNA vaccines like Pfizer or Moderna.

Neurological Side Effects

Rare neurological complications reported after COVID-19 vaccination include Bell’s palsy (temporary facial paralysis), Guillain-Barré syndrome, other neuropathies, and in very rare instances, encephalitis or myelitis.²⁰ Bell’s palsy has received the most attention. A systematic review of reported cases found that most occurred after Pfizer, followed by Moderna and other vaccines. The average interval between vaccination and onset of facial weakness was about 10 days, with a range of one to 48 days. Left-sided paralysis was somewhat more common, and most patients recovered after treatment with corticosteroids or antiviral medications.²¹

A separate meta-analysis confirmed that the majority of reported Bell’s palsy cases involved one-sided facial paralysis, occurred most often after the first dose, and responded well to standard treatment.²² It is worth noting that Bell’s palsy occurs in the general population at a background rate of roughly 15 to 30 cases per 100,000 people per year even without any vaccination, so establishing a causal link at the individual level is difficult. Large-scale surveillance studies have generally found that the rate after COVID-19 vaccination is close to or slightly above this baseline.

Menstrual Cycle Changes

Many women reported changes to their menstrual cycles after vaccination, and this concern gained enough traction that researchers investigated it formally. A systematic review found that heavier periods, irregular bleeding, and more frequent periods were the most commonly observed problems, though the reported rates varied enormously across studies.²³

A more recent meta-analysis focused on adolescents and young adults aged 12 to 25 found no overall effect of vaccination on menstrual changes as a whole. There was, however, a small but statistically significant increase in cycle length after vaccination, though when measured continuously, the increase amounted to less than a quarter of a day on average, which is clinically negligible.²⁴ The current consensus is that any menstrual disruption from COVID-19 vaccines is temporary and resolves within one or two cycles.

Autoimmune Flare-Ups

For people with existing autoimmune conditions, there is a legitimate question about whether vaccination can trigger a disease flare. A large Korean population-based study found modestly increased risks of certain autoimmune conditions after booster vaccination, including alopecia areata, psoriasis, and rheumatoid arthritis, with hazard ratios in the range of 1.12 to 1.16.²⁵ A review of autoimmune rheumatic disease flare-ups found that rheumatoid arthritis, lupus, and cutaneous vasculitis were the most frequently reported conditions among those who flared after vaccination.²⁶

A Sri Lankan study of patients with established rheumatic and musculoskeletal diseases found that roughly 5% reported flare symptoms after the first dose, rising to about 17% after the second dose. Most flares involved joint inflammation, and onset was usually within one week of vaccination.²⁷ These findings do not mean people with autoimmune conditions should avoid vaccination; rheumatology guidelines generally recommend it. But they do suggest that flares are possible, and people with active autoimmune disease should discuss timing with their physician, particularly around planned dose schedules.

The Nocebo Effect Is Real

An often-overlooked dimension of vaccine side effects is how much expectation shapes experience. Research on what is called the nocebo effect, the inverse of placebo, has shown that people who are more hesitant about vaccination report more side effects afterward. A study of older adults found that vaccine hesitancy before a booster dose significantly predicted the severity of subsequent side effects, even after accounting for other variables. The effect was stronger in men, where it was partly driven by exposure to negative information, while in women prior experience with side effects played a larger role.²⁸

Media coverage played a measurable role too. A study examining adverse-event reporting found evidence of a media-induced nocebo response: when news coverage of specific vaccine side effects increased, reports of those same side effects rose correspondingly, likely because heightened awareness and anxiety made people more attuned to normal bodily sensations and more likely to attribute them to the vaccine.²⁹ None of this means that vaccine side effects are imaginary. The immune-mediated symptoms are real. But a quantifiable layer of reported side effects is psychosomatic, and understanding that can help people approach vaccination with more accurate expectations.

Booster Doses and Mixing Vaccine Types

If you received a booster of a different vaccine type than your primary series, a so-called heterologous boost, you may have noticed stronger reactions. A meta-analysis found that heterologous boosters carried higher odds of fever, muscle pain, and fatigue compared with homologous boosters (receiving the same vaccine for all doses).³⁰ A broader systematic review and meta-analysis confirmed that mRNA vaccines were the most reactogenic booster type overall, followed by viral vector vaccines, then protein subunit vaccines, with inactivated vaccines being the gentlest. Full-dose boosters were also more reactogenic than half-dose boosters.³¹

Protein-based vaccines like Novavax showed promise as a gentler booster option. One comparative study found a lower incidence of adverse events in the Novavax booster group.³² For people who had strong reactions to mRNA boosters, these alternative platforms may be worth discussing with a healthcare provider for future doses.

How Side Effects Are Tracked

The scale of COVID-19 vaccine safety monitoring has been unprecedented. In the United States, the CDC developed v-safe, a smartphone-based active surveillance system that allowed vaccine recipients to report symptoms at daily, weekly, and monthly intervals after each dose. Text message reminders linked to web-based surveys, enabling near-real-time tracking of reactogenicity patterns across millions of people.³³ Similar systems operated in other countries, including Australia’s AusVaxSafety (established in 2014 but expanded for the pandemic) and Canada’s CANVAS, which uniquely included control groups for comparison.³⁴

These active systems are distinct from passive reporting databases like VAERS, where anyone can submit a report regardless of whether the event was actually caused by the vaccine. Active surveillance systems reach out to vaccinated people proactively, which yields a more representative picture of what side effects actually look like in the general population. Much of the reliable data on reactogenicity patterns, including the dose-dependent and demographic trends discussed throughout this article, came from these active surveillance efforts.

Allergic Reactions and PEG Sensitivity

Early in the vaccine rollout, there was concern about anaphylaxis, particularly in people with allergies to polyethylene glycol, an ingredient in mRNA vaccines. A case series of 19 patients with suspected PEG allergy who went ahead and received an mRNA COVID-19 vaccine found that 18 tolerated it without any adverse event. One patient developed mild hives that resolved with an oral antihistamine.³⁵ True anaphylaxis after COVID-19 vaccination has been reported at a rate of roughly two to five cases per million doses in large surveillance datasets, making it exceedingly rare. Standard practice at vaccination sites includes a 15- to 30-minute observation period specifically to catch and treat any immediate allergic reaction.

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