A top U.S. official said on Wednesday that Americans with known severe allergic reactions may not be candidates for Pfizer's COVID-19 vaccine, after the United Kingdom said two people with severe allergies experienced serious reactions to the Pfizer vaccine on the first day of widespread vaccinations in the country.

"My expectation is... that tomorrow this will be a part of the consideration and as in the UK, the expectation would be that subjects with known severe allergic reactions should not take the vaccine until we understand exactly what happened here," said Moncef Slaoui, the chief adviser for U.S. efforts on coronavirus vaccines.

The European Union drug regulator will also use data from Britain and other countries outside the European Union on the safety of Pfizer-BioNTech's COVID-19 vaccine before deciding whether to approve it, it said in a statement on Wednesday.

Britain's medicine regulator has advised that people with a history of significant allergies do not get the Pfizer-BioNTech vaccine after two people reported adverse reactions on the first day of the rollout on Wednesday.

Asked whether this new information could be used in the assessment of the vaccine in Europe, the European Medicines Agency said: "All available information on the quality, safety and efficacy of the vaccine will be taken into account. This will include safety data generated from the use of the vaccine outside the EU."

Meanwhile, Health Canada announced Wednesday that it has approved the use of Pfizer's COVID-19 vaccine.

Canada is the second country in the world to OK the vaccine. Doses are already being given to residents of the United Kingdom.

About 249,000 doses of the two-dose vaccine are expected to arrive by Dec. 31 and the first inoculations will be to residents and staff in long-term nursing homes.

Of Canada's deaths from the virus, about 77% were in nursing and seniors' homes as of mid-November. The death rate for Canada as of Dec. 9 is 12,932, with about 435,000 cases.

The approval came after two months of review of the Pfizer clinical trials.

"The data provided support favourably the efficacy of Pfizer-BioNTech COVID-19 vaccine as well as its safety," said Health Canada. "The efficacy of the vaccine was established to be approximately 95 percent."

"The vaccine was well tolerated by participants and has no important safety concerns."

"The benefit-to-risk assessment for Pfizer-BioNTech COVID-19 vaccine is considered favorable."