Following an advisory committee vote Thursday to recommend emergency use authorization (EUA) for Pfizer (PFE) and BioNTech’s (BNTX) COVID-19 vaccine, the U.S. Food and Drug Administration (FDA) is preparing to issue the EUA.

[Read more: FDA advisory committee recommends Pfizer/BioNTech vaccine authorization]

FDA Commissioner Stephen Hahn said Friday the agency has notified the companies of its intention to issue an EUA and alerted Operation Warp Speed officials and the U.S. Centers for Disease Control and Prevention (CDC) so they can prepare to deliver the vaccines as soon as the FDA gives the green light.

“FDA is finalizing the necessary documents to ensure that patients and providers have the information that they need to make informed decisions and to safely administer the vaccine. This is a critical part of what the agency does to promote and protect the public health,” Hahn said.

While many experts anticipated the green light to come anywhere between Friday night and Saturday, reports of pressure from the White House could affect public confidence in the approval. The Washington Post reported that President Donald Trump’s chief of staff told Hahn to approve the vaccine Friday or resign.

The time to authorize rests on finalizing details of who should receive the vaccine among specific sub-groups. Following reports of two allergic reactions in the U.K., which spurred a warning against vaccinating anyone with allergies, could result in a significant drop of qualified individuals.

Dr. Peter Hotez, a top vaccine expert and dean of the School of Tropical Medicine at Baylor College of Medicine, told Yahoo Finance Friday that he hopes the FDA can narrow down specific allergies.

“We’ve got so many people carrying EpiPens who have allergies. If they do what they did in the U.K., it would have a chilling effect. It would drop out a lot of people in the pool ... and the public perception would be damaging,” Hotez said.

In addition, the FDA must provide guidance on whether or not pregnant women should receive the vaccine. The agency has a lot to consider in a short period of time, Hotez noted.

U.S. Health and Human Services (HHS) Sec. Alex Azar told ABC’s Good Morning America Friday that the first shots could be delivered as early as Monday or Tuesday, indicating the EUA could come late Friday or over the weekend to allow time for shipments.

Hotez said he hopes shipments will begin ahead of the expected EUA so that vaccinations can begin the minute it is granted. “My recommendation would be to give the green light to ship out so they can have it all in place,” Hotez said.