Dr. Parry also points out the vaccine developers may have landed on this interval partly based on established work performed on mRNA vaccines as cancer therapies. Moreover, in a pandemic, companies were trying to make vaccines that could provide effective protection and receive approval as quickly as they could safely do so; a longer interval may have slowed things down.
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This extended booster strategy is still being used in parts of the world where COVID-19 vaccine access is limited. Dr. Parry notes the approach makes a lot of sense from a public health perspective, because it accelerates the percentage of the population that has received coverage from at least a single vaccine dose.
Dr. Calabrese doesn’t think a delayed dose approach makes sense in the U.S., where vaccine hesitancy is the key problem, not vaccine availability. Although a single mRNA vaccine dose does give some protection, she notes it is not complete, and she and her colleagues have treated patients who contracted SARS-Co-V-2 after an initial vaccine dose.
She also adds the Pfizer vaccine is 95% effective using the current two dose-timeline—very high in the context of vaccine development more broadly, making it unappealing to alter the current schedule.5
Potentially, however, the strategy may inform decisions on the need for vaccine boosters, particularly in the context of new disease variants. In such a setting, achieving a maximum antibody boost after an ideally timed dose may make a difference, particularly for elderly patients or those on immunosuppression.
“Although [a delayed second dose] approach has the potential to be beneficial for immune responses in older adults in the long term, the correct use of the strategy is unknown at present.”
Ruth Jessen Hickman, MD, is a graduate of the Indiana University School of Medicine. She is a freelance medical and science writer living in Bloomington, Ind.
- Parry H, Bruton R, Tut G, et al. Single vaccination with BNT162b2 or ChAdOx1 in older people induces equivalent antibody generation but enhanced cellular responses after ChAdOx1. Preprints with The Lancet.
- Parry H, Tut G, Faustini S, et al. BNT162b2 vaccination in people over 80 years of age induces strong humoral immune responses with cross neutralisation of P.1 Brazilian variant. Preprints with The Lancet.
- Parry H, Bruton R, Stephens C, et al. Extended interval BNT162b2 vaccination enhances peak antibody generation in older people. Preprint medRxiv.
- Gustafson CE, Kim C, Weyand CM, Goronzy JJ. Influence of immune aging on vaccine responses. J Allergy Clin Immunol. 2020;145(5):1309–1321. doi:10.1016/j.jaci.2020.03.017.
- Polack FP, Thomas SJ, Kitchin N, et al; C4591001 Clinical Trial Group. Safety and efficacy of the BNT162b2 mRNA Covid-19 vaccine. N Engl J Med. 2020 Dec 31;383(27):2603–2615.