The FDA indicated that it has no plans to make any dosing or scheduling changes to the two COVID-19 vaccines currently authorized in the US, saying doing so at this time would be “premature and not rooted solidly in the available evidence.” The statement came in response to various proposals that call for adjusting the original vaccine regimens tested in clinical trials in order to stretch out supply and give more people at least some protection against COVID-19 rather than none at all.
Ideas being floated include reducing the number of doses, extending the length of time between the first and second injections, cutting doses in half for certain populations, or “mixing and matching” different vaccines. The US has so far issued emergency-use authorizations to Pfizer and BioNTech’s BNT162b2 as well as Moderna’s mRNA-1273, both of which consist of two doses that are given 21 days and 28 days apart, respectively.
The FDA noted that each vaccine was about 95% effective in phase 3 studies at preventing COVID-19 in adults who got their second doses after the specified gap of 3 or 4 weeks, and that the available data “continue to support” such use. Moreover, the relatively few participants who did not receive their two injections at the proper intervals were typically only followed for a short period, so the FDA “cannot conclude anything definitive about the depth or duration of protection.” The agency warned that making any changes without sufficient evidence could ultimately end up being “counterproductive” to public health efforts.
Earlier this week, Moncef Slaoui, head of Operation Warp Speed, said the US was considering giving half doses of Moderna’s vaccine to adults under the age of 55, citing earlier study data where it appeared to trigger effective antibody responses in this group. He also suggested this would be “a more responsible approach” than the UK’s recent decision to prioritize giving more people a single injection of COVID-19 vaccine, rather than reserving second doses for those who already received their first.
UK experiments with delays, mix-and-match
Officials in the UK, where so far BNT162b2 and AstraZeneca’s AZD1222 have been authorized, claim that allowing second doses to be given as many as 12 weeks after the first one will help maximise immunity in the population, at least in the short term, an idea that may be catching on in other countries, including Germany. However, Pfizer has warned that people risk not being sufficiently protected if the gap between vaccine doses is longer than recommended.
Meanwhile, the UK has also been criticized for a decision to allow people in some cases to be given doses of different COVID-19 vaccines, rather than two of the same one, although an official at Public Health England stated that this would only be permitted in “extremely rare” circumstances.