(CNN)Pfizer and the US Food and Drug Administration are "negotiating" what information should be on the label for the company's coronavirus vaccine, considering two people in the UK had adverse reactions to the shots this week, according to the acting chairman of an FDA vaccine advisory committee.
"They're going back and forth about what wording should be on the label -- that's what's happening right now," said Dr. Arnold Monto, acting chairman of the FDA's Vaccines and Related Biological Products Advisory Committee, which voted Thursday to recommend authorization of the vaccine.
In the UK, health authorities have warned that people with a "significant history of allergic reactions" should not be given the vaccine.
It's not clear if the FDA will follow suit.
"There are a lot of people with allergies - how do we define it?" Monto said.
A source close to the White House coronavirus task force told CNN that authorization could come by the end of the day.
In an interview Friday, US Health and Human Services Secretary Alex Azar made reference to FDA's negotiations with Pfizer.
"In the next couple of days, probably, as we work to negotiate with Pfizer the information doctors need to prescribe it appropriately, we should be seeing the authorization of this first vaccine," Azar said on ABC's Good Morning America. "And so we could be seeing people getting vaccinated Monday, Tuesday of next week."
Monto said the allergic reaction issue, and others, need to be worked out.
"The FDA will move this out as soon as they can, but they have to be sure the label and everything else are reflecting their negotiations with Pfizer," he said.
There's also a question of whether the label should contain any special information for 16- and 17-year olds, considering that relatively few people that age were involved in Pfizer's vaccine clinical trials, Monto said.
"I think they're going to issue the authorization down to age 16 -- that's pretty clear -- but I don't know if they're going to require that the label say that the data for people that age is not as extensive," Monto said. "You know what labels are like -- they're very long, so you can only imagine what they're going through right now."
Get CNN Health's weekly newsletter
Sign up here to get The Results Are In with Dr. Sanjay Gupta every Tuesday from the CNN Health team.
It might not be decided before an EUA, but the agency and the company are also likely discussing an issue that is not related to labelling: When should clinical trial participants who received a placebo be offered the vaccine?
In Pfizer's clinical trials of more than 40,000 study subjects, half the participants were given the vaccine, and half were given a placebo, or a shot of saline that does nothing.
Neither the study participants nor the doctors running the trials know who received the vaccine and who received the placebo. It's unclear when they should be told, and when the participants who received the placebo should be offered the vaccine.
If the participants who received the placebo get vaccinated, it interferes with Pfizer's ability to study any differences between the two groups in the future.
Monto said the agency would like to resolve this issue before authorizing the vaccine.
"They want to move as fast as they can, but they have to get all these things decided before the vaccine can be approved," he said.
