Before they are offered to the public, vaccines are given to thousands of people to test whether they safely prevent serious illnesses. After vaccination, many people roll down their sleeves and go about the rest of their day, experiencing no issues at all. Some have a sore arm or, with certain vaccines, tiredness or a general feeling of being under the weather. This is generally mild and short-term, lasting a day or two.
Independent regulators determine whether to approve a vaccine, based on the balance between these adverse events and the benefit of protection against a serious illness. All medical interventions, from cholesterol medicines to chemotherapy and surgery, come with potential downsides which a patient and clinician must consider: removing a brain tumour is risky, but doing nothing may be worse.
For vaccines, the standard is particularly high as vaccines are given to the whole population, including infants, vulnerable individuals and people who are in good health. That is why clinical trials, using in healthy individuals, are studied closely before experts give the green light for a new vaccine.
However, after approval, when millions of people get vaccinated, rare problems can arise: a one in a million event that was not picked up in a trial of 5,000 people. That’s where ‘post-marketing surveillance’ comes in. Individuals and health professionals can report suspected symptoms to regulators. These might include a rash or dizziness soon after vaccination, or a less common health issue weeks after vaccination.
The regulator looks for patterns in these reports, and considers plausible connections between vaccination and the reported issue, to determine whether the vaccine may be the cause. In some cases, the vaccine is the cause and the regulator may pause its use or limit its advice on who can be offered the vaccine. In other instances, the vaccination is unrelated to the suspected issue.