Pfizer plans for third dose: questions over population need?

Simon Collins, HIV i-Base

One of the key emerging issues has been over the need for a third vaccine dose.

Accumulating reports, show the benefit of a third vaccine in some vulnerable populations that fail to generate optimal antibody responses after two shots. [1, 2, 3]

This is easy to support. It is in line with national vaccine programmes to ensure protection in people at highest risk of severe COVID-19.

But this is different to the proposal for universal use of a booster dose on a population level. Offering vaccines to all adults in rich countries, not only delays first course programmes to the rest of the world but involves the commercial conflict of large profits for manufacturers.

Over the last month, Pfizer has reported plans to apply to the FDA for coverage of a third dose, though anecdotal rather in a formal press release. This would extend the indication from a two dose to a three dose schedule. [4]

The evidence to support this need is limited, largely based on waning antibody protection and reports from the vaccine programme in Israel. A pre-review analysis from the original phase 3 study also reported that after six months, protection against symptomatic infection dropped from 96% to 84%. [5]

But this is for symptomatic infection and doesn’t report reduced efficacy against hospitalisation. This paper, not yet peer reviewed, says further follow-up to two years is needed before knowing whether a third booster dose is needed.

So far the US CDC and FDA have announced that there is currently no need for a universal third dose. [6]

While data on the safety on a third dose would help individuals who didn’t generate immune responses to two vaccines, the clinical need for the third dose is likely to be more complicated than waning antibody levels, especially if the proposed booster doses have not been developed to produce strong protection against variants.

Also, current data, including from the UK, show current vaccines are currently effective at reducing hospitalisation against the Delta variant, even if transmission and mild symptoms still occur. [7]


  1. Third dose of mRNA vaccine improves antibody responses in kidney transplant recipients, HTB (August 2021).
  2. Third vaccine dose increases immune response to 68% in French transplant recipients. HTB (July 2021).
  3. Third COVID-19 vaccine dose in US cohort of people on immune-suppressing treatment: safety and ethical issues. HTB (1 July 2021).
  4. Neergaard L. Pfizer to seek FDA authorization for third, booster dose of its Covid-19 vaccine. (8 Jult 2021).
  5. Thomas SJ et al. Six month safety and efficacy of the BNT162b2 mRNA COVID-19 vaccine. MedRxiv. doi: 10.1101/2021.07.28.21261159. (28 July 2021).
  6. FDA news release. Joint CDC and FDA statement on vaccine boosters. (8 July 2021).
  7. Efficacy of Oxford/AZ and Pfizer vaccines against Delta variant in the UK. HTB (August 2021).

This article was first posted on 28 July 2021,

Links to other websites are current at date of posting but not maintained.