The Food and Drug Administration said that it will be investigating reports of some recipients of Pfizer’s coronavirus vaccine candidate experiencing severe allergic reactions. The agency clarified that the rollout of the vaccine will remain unhindered while it conducts the probe.

Two of the healthcare workers that had severe allergic reactions to the vaccine were from the UK, while a third worker was from Alaska. A hospital worker reportedly experienced a less serious reaction to the vaccine.

The FDA said that it is closely monitoring the conditions of the affected recipients. The agency said that it will be working with the Centers for Disease Control and Prevention to investigate the claims.

The FDA said that it is also working with Pfizer to help it update its fact sheets and prescribing information. It also assured the public that all facilities that will be administering the vaccine will be required to have the ability to immediately treat any severe allergic reactions.

“While the totality of data at this time continues to support vaccinations under the Pfizer [emergency use authorization] without new restrictions, these cases underscore the need to remain vigilant during the early phase of the vaccination campaign,” the FDA said.

The three people that had an adverse reaction to the vaccine reportedly experienced anaphylaxis — a sudden allergic response that is easily remedied with medication. Pfizer reiterated the safety of its treatment, stating that the vaccine had passed strict safety reviews and was tested in trials involving tens of thousands of people.