The National Institutes of Health is reportedly planning to conduct a clinical study to determine why people are having severe allergic reactions to Pfizer's COVID-19 vaccine.

According to the Washington Post, during a press briefing on Monday, Moncef Slaoui, the chief science adviser for Operation Warp Speed, said they are seeking volunteers who've had allergic reactions to the vaccine, so doctors can identify why a rare event known as anaphylaxis is happening.

The NIH also plans to study the Moderna vaccine as well, Bloomberg reported.

Last week, health authorities in the U.K. said they were investigating after two people had "adverse reactions" to the Pfizer vaccine on the first day of the country's mass vaccination program.

In the U.S., two healthcare workers in Alaska reported having an allergic reaction, and on Friday, the FDA announced five people also had an "adverse reaction" to the vaccine.