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How safe are COVID-19 vaccines?

Given the speed at which COVID-19 vaccines have been developed, it is understandable that people want to know whether they are safe. So what measures have been put in place to ensure the safety of these new vaccines?

  • 4 February 2021
  • 5 min read
  • by Priya Joi
How safe are COVID-19 vaccines?
How safe are COVID-19 vaccines?


Nearly a year after the COVID-19 pandemic began, several vaccines have now been rolled out across the world, including some that use new RNA technology that has never been approved for use on humans before. Although all these vaccines have been produced at record speed, with processes run in parallel to save time, there have been many checks and balances to ensure their safety, including being subject to the same scientific and regulatory rigour as any other vaccine.

Any possible risks that may exist are considerably lower than those associated with COVID-19 infection, and vastly outweighed by the benefits of protecting people and preventing the virus from spreading.

Ensuring safety

A candidate vaccine goes through several stages before it can be given to people – right from the exploratory science, to the pre-clinical testing (often on animals), then clinical development (which includes three phases of human trials), and finally regulatory review and approval, manufacturing and quality control.

In pre-clinical studies, a vaccine is tested to see whether it is toxic and how it reacts with the body – this is to identify a safe dose before testing the vaccine candidate in people.

Human trials are designed to spot side effects – these are not the same as temporary short-term reactions such as a headache, sore arms, fatigue, chills and fever, which are not uncommon in other vaccines or injections, and are usually not harmful in the long term. Genuine side effects means anything that is long-lasting and potentially dangerous.

New technology

Two of the new vaccines developed by Pfizer-BioNTech and Moderna use mRNA technology. The advantages of mRNA vaccines are numerous – they are fast to develop, cheaper and easier to manufacture than more traditional vaccines. Although RNA vaccines have not been used on the public before, this technology has been carefully developed for over a decade and there are many reasons to be reassured that they are safe.

So how does an mRNA vaccine work? RNA (ribonucleic acid) is related to DNA, and mRNA is messenger RNA, part of our genetic code that tells cells what proteins to make. The COVID-19 vaccines exploit this by sending a copy of the messenger RNA belonging to the coronavirus into our cells instructing them to make spike proteins similar to those found on the surface of the COVID-19 virus. This works because the spike protein triggers a strong immune reaction in our bodies.

But the viral RNA does not tamper with our own genetic code because it does not enter the nucleus at the heart of our cells, and so does not come into contact with our DNA. Also RNA disintegrates within about 72 hours.

Side effects

During clinical trials, the Pfizer-BioNTech vaccine was given to half of a total of 43,500 people, and the Moderna vaccine to around 15,000 of 30,000 people in the trial. Throughout the phase 1, 2 and 3 trials assessing safety, no major side-effects have been reported so far.

Oxford/AstraZeneca’s candidate is a vector vaccine, which also works by inserting part of the genetic code of the virus into cells instructing them to make spike proteins. But unlike mRNA vaccines these instead use a harmless virus to deliver it. In clinical trials of this vaccine there were very rare reports of events associated with inflammation of the nervous system, but it is not clear whether those events were related to the vaccine.

The Task Force for Global Health’s Brighton Collaboration is a group of more than 750 vaccine experts working on vaccine safety, including for COVID-19 vaccines. In partnership with the Coalition for Epidemic Preparedness Innovations (CEPI) – which, together with Gavi and the World Health Organization (WHO) is driving forward COVID-19 vaccine development – the Brighton Collaboration convened global experts to draft lists of potential adverse events of special interest for COVID-19 vaccine candidates which are reviewed by WHO.

This is just one part of the routine monitoring carried out on use of a new vaccine in the population, to watch out for any unexpected side effects. This includes a wide range of possible side-effects, including any respiratory complications (such as pneumonia or acute respiratory distress syndrome), as well as cardiac (including cardiogenic shock, cardiomyopathy or coronary artery disease), acute renal and hepatic injury, or sepsis.

Myths and misinformation

As with any drug or vaccine, separating fact from fiction is critical and can be increasingly hard to do with the amount of noise surrounding the safety of COVID-19 vaccines. Rumours circulating on the internet and on social media platforms include, for example, ones claiming that the new vaccines can affect fertility, despite there being no evidence. More importantly, there is no biological possibility that COVID-19 vaccines could affect fertility.

Nor can the vaccines actually give you COVID-19, and there is no biologically plausible mechanism through which they could do so. While there have been anecdotal reports of people appearing to get COVID-19 after the first dose of the vaccine, it is most likely that these patients already had the virus incubating before their shot. Or they were part of the small percentage of people in whom the vaccine failed to trigger an immune response, and then subsequently became infected. 

Ultimately safety is paramount throughout the entire vaccine development and the regulatory approval process. Moreover, any possible risks that may exist are considerably lower than those associated with COVID-19 infection, and vastly outweighed by the benefits of protecting people and preventing the virus from spreading.