COVID-19: Vaccine clinical trials and testing

Find out how the COVID-19 vaccines were developed, and what the data tells us and how the vaccines are monitored.

Last updated: 19 August 2021

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How the COVID-19 vaccines were developed so quickly

There’s never been this level of global collaboration amongst scientists and governments in vaccine development. This has improved the speed of its development and the launch of clinical trials around the world.

Researchers were able to use their knowledge of other coronaviruses and vaccine development to give them a head start.

Clinical trials were also able to recruit large numbers of volunteers faster than usual because of the worldwide interest and concern about COVID-19. Some clinical trials could be done at the same time instead of one after the other. This meant they could quickly determine whether the vaccine was effective in a short amount of time – under normal circumstance this could take many months or even years.

Large manufacturing plants have been developed, so vaccines can be produced faster and on a larger scale than was previously possible.

The vaccines have been developed very quickly but without taking any shortcuts in the necessary processes or compromising safety.

How the vaccine works


What the clinical trials assessed

Clinical trials for a COVID-19 vaccine are designed to assess the safety and efficacy of the vaccine. The Pfizer vaccine (Comirnaty) has been assessed in global studies across three phases.

  • Phase one and two assessed the safety and immunogenicity (the immune response after each dose) of different dose levels of the vaccine in a small population.
  • Phases two and three assessed the safety and efficacy of the vaccine against symptomatic COVID-19 after two doses of the chosen level, given 21 days apart, in a larger population.

In addition, early findings from a small number of well-designed studies, show that an extended gap of 6 weeks or more between doses of the Pfizer vaccine gives at least an equally robust immune response.

Clinical trials in young people

On June 30 2021, Medsafe published its provisional approval of the Pfizer/BioNTech vaccine for 12- to 15-year-olds in New Zealand. 

Medsafe’s experts only grant consent for a vaccine to be used in New Zealand/Aotearoa once they are satisfied it has passed required levels of safety and effectiveness. 

The Pfizer/BioNTech vaccine has already been approved for 12- to 15-year-olds in Canada, the USA, Europe and Japan. 

Pfizer’s study in 12- to 15-year-olds looked for signs of a strong immune response to the vaccine. Pfizer reported 100 percent efficacy in this age group (a higher antibody response than those studied in the 16- to 25-year-old age group). 

The process used to assess the safety of the vaccine is the same robust process used to assess other medicines, like the flu vaccine. 

Side effects and effectiveness

Learn more about the known side effects of the Pfizer vaccine and how it protects us:

Vaccine side effects and reactions
Vaccine effectiveness and protection


Participants in the clinical trials

Across the clinical trials, the Pfizer vaccine (Comirnaty) was studied on about 44,000 participants. Half of these people received the vaccine and half received a saline placebo.

Participants had a range of different ethnicities, ages, sexes and underlying health conditions.


Ongoing clinical trials

Ongoing clinical trials, safety monitoring, and real-world data from COVID-19 vaccination programmes worldwide provide us with useful information. This includes the long-term safety and benefits of the vaccine.

Monitoring clinical trial participants

To understand the long-term efficacy and safety of the vaccine, participants in the clinical trials are tracked for at least two years from their second dose of the Pfizer vaccine.

Participants will:

  • have their health monitored
  • attend regular follow-up visits.

This is closely observed by Pfizer/BioNTech and an independent group of experts called the Independent Data Monitoring Committee. They decide if a clinical trial can continue.

Reviewing new data and evidence

Regulatory authorities, like Medsafe, have procedures to closely review all new data and will update their recommendations as this becomes available.


Real-world data

We’re continually reviewing new data as it’s being reported.

Medsafe is closely monitoring real-world data from the vaccination programmes across the world, in particular on the safety of the Pfizer vaccine (Comirnaty). This will add to the clinical data we receive from Pfizer/BioNTech. This includes:

  • the overall safety profile of the vaccine
  • any reported reactions (the frequency, the severity, and any previously unknown reactions)
  • the effectiveness of the vaccine overall and in certain groups.

Reported side effects (adverse events) after receiving the COVID-19 vaccine – Medsafe
COVID-19 vaccine safety monitoring process – Medsafe  

How many people have had the Pfizer vaccine

The number of people receiving this vaccine continues to grow significantly every day. For up-to-date numbers visit the Our World in Data website.

Worldwide statistics and research on COVID-19 vaccinations – Our World in Data

How many people have been vaccinated in New Zealand

See the data and statistics about the rollout of COVID-19 vaccines in New Zealand.

COVID-19 vaccine data

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