How do scientists make sure new vaccines are safe to use?

Ideally, adverse events are compared between participants who receive the vaccine and those who receive a placebo or comparator vaccine. This helps to distinguish after effects (AEs) directly related to the vaccine from those related simply to the injection or symptoms seen in the wider community at the time.

In the most rigorous clinical trials, participants and staff are ‘blinded’ to the participants study allocation. Neither know which participants have received the new vaccine and which have received the placebo/comparator vaccine (‘double blinded’).

Blinding helps to ensure that participants and study staff don’t wrongly assume a symptom is caused by a vaccine when it is not.

For example, in the diagram below of a randomised controlled double-blind trial assessing the safety of a vaccine against a placebo, the results suggest arm pain is likely to be an AE related the vaccine, whereas fatigue may not be specific to the vaccine.

If you need a text version of the image above, this is available as a PDF.

Reporting AEs

Strict reporting requirements are in place so that any serious AEs — for example those requiring admission to hospital or resulting in disability or death — are carefully and rapidly assessed to understand their relationship to a new vaccine.

Meningitis, for example, in a trial participant six months post-vaccination is a severe AE, but very unlikely to be related to the trial vaccine and so not of concern.

However, if a participant developed meningitis three days after vaccination this could be a concern, especially if the AE was seen in more than one participant.

Study doctors are required to report these severe AEs to regulatory authorities immediately, who may advise the trial is paused and no more participants vaccinated until more information is collected. This approach is routine and further details can be understood from this article regarding a halt on further vaccinations in the University of Oxford COVID-19 vaccine trial.

Phases 2 and 3

Provided there are no safety concerns, clinical trials progress to Phase 2 and Phase 3, increasing the numbers of participants.

Trials are undertaken to assess the safety of the vaccine in different doses, different dosing schedules and in different age groups and patient groups.

In order for a new vaccine to be licensed for use, a dossier of all safety data from clinical trials must be submitted to the regulatory authority. Only if the vaccine is deemed to be safe will a license be granted.