“It may be a coincidental case, but there is a need to gather more information and make an assessment in upcoming working groups.”
Pfizer officials in Japan were not immediately available for comment. Pfizer said in November the efficacy of its vaccine was consistent across age and ethnic groups, and that there were no major side effects, a sign that the immunisation could be employed broadly around the world.
Global health authorities have praised the fast development of safe and effective Covid vaccines, but have warned people with serious underlying health conditions to take medical advice first.
Japan became the last member of the Group of Seven leading industrialised nations to begin its vaccination drive, on February 17.
It has so far received three shipments of vaccine developed by Pfizer and BioNTech.
Japan officially approved Pfizer's vaccine last month, the first such approval in the country as it steps up efforts to tame infections in the run-up to the Summer Olympics.