This evidence synthesis has been compiled by members of the CITF Secretariat and does not necessarily represent the views of all CITF members.

By Alexis Palmer-Fluevog 

Researchers from the University of Birmingham recruited a cohort of older adults to compare the immune response from the standard dosing schedule to that of an extended interval dosing schedule for the Pfizer-BioNTech vaccine. In this pre-print, therefore not yet peer-reviewed, the researchers suggest that a dose interval of 12 weeks results in an antibody response 3.5-fold higher than the currently recommended dose interval of three weeks. This has important implications for timing of vaccine rollout worldwide.

Many COVID-19 vaccines are given in two doses: the first shot kick starts an immune response and the second dose strengthens it. Previous trials and real-world scenarios have shown that the Pfizer-BioNTech vaccine is very effective against COVID-19. The vaccine’s current recommendations are to space out the two doses three weeks apart, however due to a multitude of factors, many countries, including many Canadian provinces, have decided to extend the dosing interval from three weeks to 16 weeks.

Key points:

  • Peak antibody responses were enhanced (3.5-fold higher) in participants when the second dose was delayed to 12 weeks, although cellular responses were lower (3.6-fold lower).
  • Extended interval vaccination may therefore offer the potential to enhance and extend humoral immunity.

The study group was composed of 172 people aged over 80, living independently, and recruited through primary care physicians. All participants were vaccinated with the Pfizer-BioNTech vaccine with either a standard 3-week interval or an extended interval of 11-12 weeks after the first dose. Following blood draws at 2-3 weeks and at 8-9 weeks after the second dose, analysis showed that both standard-interval and extended-interval vaccination regimens elicit strong antibody responses, but peak values are 3.5-fold higher with the extended interval regimen.

With vaccine demand high and supply low, the United Kingdom, Canada and other countries opted to delay the second dose from 12-16 weeks after the first in an effort to expand “first-dose” immunity among a greater proportion of the population. While the results presented here are reassuring, the authors point out that they are specific to the Pfizer vaccine. In addition, countries need to consider whether the variants circulating in their region might raise the risk of infection with partial protection.

Parry HM, Bruton R, Stephens C, Brown K, Amirthalingam G, Hallis B, Otter A, Zuo J, Moss P. Extended interval BNT162b2 vaccination enhances peak antibody generation in older people. medRxiv. 2021 May 17. doi: 10.1101/2021.05.15.21257017