As growing numbers of younger adults get vaccinated against covid-19, social media is awash with conversation about side effects, which appear to be more common in young people. What kind of side effects can people expect, how can they be distinguished from signs of the rare blood clot syndrome linked to some vaccines, and what do they mean for people’s immunity?

What side effects may people experience after a covid-19 vaccine?

All vaccines can cause pain and swelling at the injection site, as well as more widespread, or “systemic”, effects. According to the National Health Service in the UK, these can include fever, fatigue, headache, muscle and joint pains, nausea and chills – when people feel cold without apparent cause – and tend to happen in the first day or two after the jab. They shouldn’t last longer than a week. The impression of covid-19 vaccinators is that between a third and a half of people experience these systemic effects in some form, but most will be mild, says Paul Morgan at Cardiff University in the UK, who is a member of the British Society for Immunology’s covid-19 taskforce.

Why do side effects occur?

Most coronavirus vaccines work by forcing the body to make the coronavirus spike protein, which is the part of the virus that allows it to bind to and invade cells. This then triggers an immune response. However, it takes a few days for the body to start producing the spike protein. This means that any immediate side effects you experience are probably a response to other vaccine components, such as the liposome shell used to deliver the spike protein mRNA in the Pfizer/BioNTech jab or the adenovirus that contains spike protein DNA in the Oxford/AstraZeneca one. Immune cells respond to these unfamiliar substances in the arm muscle by releasing signalling chemicals called cytokines to activate other parts of the immune system. The result is systemic inflammation, leading to aching, tiredness and in some cases a fever. “It’s a danger signal,” says Morgan. “The body needs to be alerted that something’s going on and primed to respond. It brings in the right sorts of cells to clear the damage or pathogen.”

What are the signs of a dangerous blood clot?

The covid-19 vaccines made by Oxford/AstraZeneca and Johnson & Johnson have been linked with rare cases of a syndrome involving unusual blood clots, often in the brain, coupled with low levels of platelets, small particles in the blood that cause clotting. According to advice from the NHS, people who have had either of these vaccines should consult a doctor if they have a severe headache, leg swelling, abdominal pain or shortness of breath between four days and four weeks later. A rash that looks like small bruises or bleeding under the skin could also be a sign of low platelets.

It is important to remember that the risk of adverse side effects from these vaccines is very low. Francis Collins, director of the US National Institutes of Health, told NBC on Sunday that while blood clotting issues sound scary, the risk of a person experiencing adverse side effects from the Johnson & Johnson vaccine is around a “thousand times less likely” than experiencing adverse effects from aspirin. The overall rate of blood clotting was four cases per million people who have received the Oxford/AstraZeneca vaccine in the UK.

For people under 30, the chance of the Oxford/AstraZeneca vaccine causing the clotting reaction is a little greater than the risk of severe illness from covid-19. For this reason, several countries have restricted its use in younger people.

Are side effects more common in younger people?

Results from trials of the vaccines from Pfizer/BioNTech, Oxford/AstraZeneca and Moderna suggest that younger adults are more likely to experience injection site and systemic reactions. This could be because older people have a weaker inflammatory response, as the immune system’s power wanes with age, says Morgan. Many countries initially prioritised vaccinating older people and moved to younger adults later so this could be why social media recently seems to be awash with reports of systemic side effects.

Are side effects worse with the Oxford/AstraZeneca vaccine?

A study of users of the Covid Symptom Tracker app showed that about three in 10 reported systemic effects after the Oxford/AstraZeneca jab, compared with about one in 10 with the Pfizer/BioNTech vaccine. But other factors may be responsible, such as differences in who received the two jabs and the likelihood of reporting side effects. It is hard to know for sure as there are no trials yet published that directly compare the different vaccines, using the same definitions of side effects. There have been similar rates of side effects reported through the UK’s Yellow Card system, of about three to six reports sent in per 1000 doses given for both jabs.

Do side effects mean you have a stronger immune response?

Yes – and no. In a study of people who received the Pfizer/BioNTech or Moderna vaccines, there were hints that those who had systemic effects after the second dose had higher levels of antibodies. But they didn’t have higher levels of a type of memory cell that triggers the production of antibodies, which helps maintain immunity to covid-19 in the months following vaccination. Morgan says people have great variability in their reactions to vaccines and those who notice no systemic effects have nothing to worry about. People will respond differently to inflammation, for instance, he says, but having no side effects doesn’t mean you haven’t built an immune response.