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COVID-19: Immunology, vaccines and epidemiology
COVID-19: Immunology, vaccines and epidemiology

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1.4 Vaccine testing

Vaccines undergo rigorous trials before they are released for general use. The one exception to this rule is where an infection is very dangerous or uncontrolled and it is necessary to put a vaccine into the field as quickly as possible. This was seen with an Ebola virus vector vaccine, in helping control outbreaks of Ebola in the Democratic Republic of Congo and Zaire. Where mortality from a virus infection is high, there is more tolerance of adverse reactions against the vaccine, and the normal extended testing programs can be abbreviated.

A normal testing program is carried out in four phases.

  • Phase-1 examines basic safety of the vaccine in healthy volunteers.
  • Phase-2 expands the initial trial to a larger and more diverse group of individuals (older, younger, different ethnic groups, etc.).
  • Phase-3 determines whether the vaccine is effective in a large cohort (thousands of people) in real-world conditions.
  • Phase-4 trials look for any long-term effects of the treatment and may extend over many years.

[Your laboratory investigation this week is to carry out antibody testing on volunteers in a phase-1 vaccine trial.]

Due to the urgency to develop vaccines against COVID-19, volunteers were recruited as quickly as possible and where possible phase 1-3 trials were overlapped to reduce the time before results became available. Also, the vaccination schedules and doses were chosen in these trials as a best estimate of what would produce a good antibody response. Dosing and schedules were later refined pragmatically and as more data on effective schedules became available. For example, the initial schedule for the Pfizer/Biontech mRNA vaccine was 2 doses given 21 days apart. This schedule was approved in December 2020. However, when it came to roll out the vaccination programme, with limited supplies of vaccine it was thought that greater protection could be given to the population by immunising more people, but spacing out the first and second doses by 6-8 weeks. As it happened, the longer gap between doses actually produced a slightly better antibody response, and the aim of protecting more people sooner was epidemiologically sound.