FDA issues dire COVID-19 vaccine warning - Don't delay, skip or mix doses

The US FDA has warned that skipping the second COVID-19 vaccination, mixing drugs, delaying when patients get the second dose, or generally straying from the current immunization guidance could "run a significant risk of placing public health at risk." The alert comes amid reports that some areas are considering delaying the second of the two doses required for coronavirus vaccination, or mixing together supplies from different pharma companies such as Pfizer and Moderna, to maximize how many people can be immunized.

Currently there are several different COVID-19 vaccinations that the US Food and Drug Administration has granted Emergency Use Authorization (EUA) for deployment during the pandemic. Most rely on two consecutive injections, given at specified intervals. For Pfizer and BioNTech's vaccine, for example, the second dose is meant to be given 21 days after the first. Moderna's COVID-19 vaccine doses, meanwhile, are separated by 28 days.

"We have been following the discussions and news reports about reducing the number of doses, extending the length of time between doses, changing the dose (half-dose), or mixing and matching vaccines in order to immunize more people against COVID-19," the FDA said in a new statement. While the agency agrees that these are all valid possibilities worthy of evaluation in clinical trials, it also points out that they're not a good reason to alter the authorized dose regimen on which the EUAs were granted.

"Without appropriate data supporting such changes in vaccine administration," the FDA warns, "we run a significant risk of placing public health at risk, undermining the historic vaccination efforts to protect the population from COVID-19." Of particular concern is how the data is being misinterpreted.

"In the phase 3 trials, 98% of participants in the Pfizer-BioNTech trial and 92% of participants in the Moderna trial received two doses of the vaccine at either a three- or four-week interval, respectively," the FDA explains. "Those participants who did not receive two vaccine doses at either a three-or four-week interval were generally only followed for a short period of time, such that we cannot conclude anything definitive about the depth or duration of protection after a single dose of vaccine from the single dose percentages reported by the companies."

Among the challenges the COVID-19 vaccination roll-out faces is the requirement for two doses per individual. One suggestion – made both in some US states as well as abroad – has been that by delaying the second dose, or skipping it altogether, the limited supplies of the drugs could be shared among more people. In some cases, that's been based on faulty assumptions about just how effective a single dose of the vaccine might be.

"We know that some of these discussions about changing the dosing schedule or dose are based on a belief that changing the dose or dosing schedule can help get more vaccine to the public faster," the FDA says. "However, making such changes that are not supported by adequate scientific evidence may ultimately be counterproductive to public health."

One danger, for example, is that those given only a partial vaccination might assume they have more resistance to COVID-19 infection than they actually do. That could lead to them engaging in more risky behavior than is advisable, adding extra load to already overwhelmed healthcare providers, and possibly further spreading coronavirus among more people should social distancing be compromised.

Moderna announced earlier this week that it expected to be able to produce more COVID-19 vaccine doses than initially believed. However the news came as several states reported falling behind in meeting immunization goals, emphasizing the difficulties faced in actually distributing the drugs as they became available.