Two NHS staff members who received the Pfizer/BioNTech vaccine on the first day of the mass vaccination program suffered an allergic reaction, the NHS in England confirmed on Wednesday.
It is understood both staff members have a significant history of allergic reactions - to the extent where they need to carry an adrenaline auto-injector with them.
They were among thousands who received the vaccine on Tuesday.
The Medicines and Healthcare products Regulatory Agency has given precautionary advice to NHS trusts that anyone who has a history of "significant" allergic reactions to medicines, food or vaccines should not receive the vaccine.
The NHS in England said all trusts involved with the vaccination program have been informed.
This means that anyone scheduled to receive the vaccine on Wednesday will be asked about their history of allergic reactions.
"As is common with new vaccines, the MHRA have advised on a precautionary basis that people with a significant history of allergic reactions do not receive this vaccination after two people with a history of significant allergic reactions responded adversely yesterday," NHS national medical director Professor Stephen Powis.
"Both are recovering well."
The pair developed symptoms of "anaphylactoid reaction" shortly after receiving the vaccine, but both recovered after the appropriate treatment.
Pfizer said the vaccine was "well tolerated" during the trials with "no serious safety concerns".
"We have been advised by MHRA of two yellow card reports that may be associated with allergic reaction due to administration of the Covid-19 BNT162b2 vaccine," a spokeswoman said.
"As a precautionary measure, the MHRA has issued temporary guidance to the NHS while it conducts an investigation in order to fully understand each case and its causes.
"Pfizer and BioNTech are supporting the MHRA in the investigation.
"In the pivotal phase 3 clinical trial, this vaccine was generally well tolerated with no serious safety concerns reported by the independent Data Monitoring Committee.
The trial has enrolled over 44,000 participants to date, with more than 42,000 receiving a second vaccination.