In late 2020 and early 2021 people started saying that the Covid-19 vaccines skipped standard animal trials before they were tested on humans.

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There is a known and expected process for the development of a new vaccine to follow, and this includes animal testing before it is given to humans. The University Of Oxford details these steps as follows:

  • Reviewing what has been done before.
  • Theoretical development or innovation: coming up with a new idea, or a variation on an existing idea.
  • Laboratory testing and development. This involves ‘in vitro’ testing using individual cells and ‘in vivo’ testing, often using mice. The vaccine has to pass rigorous safety tests at this stage, and demonstrate that it works in animals.
  • Phase I study – an initial trial involving a small group of adult participants (up to 100 people). This is carried out to make sure that the vaccine does not have major safety concerns in humans, and also to work out the most effective dose.
  • Phase II study – a trial in a larger group of participants (several hundred people). Phase II trials check that the vaccine works consistently, and look at whether it generates an immune response. Researchers also start looking for potential side effects.
  • Phase III study – a trial in a much larger group of people (usually several thousand). Phase III trials gather statistically significant data on the vaccine’s safety and efficacy (how well it works). This means looking at whether the vaccine generates a level of immunity that would prevent disease, and provides evidence that the vaccine can actually reduce the number of cases. It also gives a better chance of identifying rarer side effects not seen in the phase II study.
  • Licensing – expert review of all trial data by the UK government (through the Medicines and Healthcare products Regulatory Agency – MHRA)  At this stage the regulators check that the trials show that the product meets the necessary efficacy and safety levels. They also make sure that, for most people, the product’s advantages far outweigh the disadvantages.
  • Phase IV studies – post-marketing surveillance to monitor the effects of the vaccine after it has been used in the population. These may be requested by a regulatory body, or carried out by the pharmaceutical industry.


Two of the three main vaccines developed for Covid-19 did skip the Laboratory testing phase and instead went straight to Phase 1. This was the Pfizer Biontech vaccine and the Moderna vaccine. They chose to instead run the animal trials in parallel with their Phase 1 human trial, thus removing the rigorous animal testing safeguard.

The other vaccine developer did not skip this phase, this was AstraZeneca.

To date none of the vaccines have been licensed or approved and instead have an emergency use status from the Food and Drug Administration (FDA) as well as national governments.






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