Blood clots have been reported within a short time after AstraZeneca Covid-19 vaccination, but such cases were very rare, according to the Department of Disease Control (DDC).
Dr Thongchai Keeratihattayakorn, the DDC director-general, acknowledged the issue on Thursday, saying AstraZeneca’s Covid vaccine was administered at a time when there was limited global awareness of potential adverse effects, as the vaccines had been developed very quickly.
The only other available vaccine at the time, he said, was the inactivated one from Sinovac, but it was not widely accepted in the West. As a result, visitors were required to be inoculated with the vaccine developed by AstraZeneca, as mRNA vaccines were not yet available.
“Instances of blood clots after AstraZeneca shots were observed between five and 42 days post-inoculation,” said Dr Thongchai. “Any occurrences of blood clots beyond this timeframe should not be attributed to the vaccine, and recipients of the AstraZeneca vaccine need not worry.”
The issue came to light this week, when the multinational drug company admitted for the first time, in court documents in a UK class-action suit, that the vaccine can “in very rare cases”, cause Thrombosis with Thrombocytopenia Syndrome. The condition is characterised by blood clots and a low blood platelet count in humans.
However, regulators have acknowledged for some time that “unusual blood clots with low blood platelets should be listed as very rare side effects” of the AstraZeneca vaccine.
The European Medicines Agency, in a report dated April 7, 2021, said most of the cases reported had occurred in women under 60 years of age and within two weeks of vaccination.
“Based on the currently available evidence, specific risk factors have not been confirmed,” it said.
In any case, it said, the benefits of taking the vaccine were seen to outweigh the risks.
In Thailand, about 48 million doses of the AstraZeneca vaccine were administered to about 20 million recipients, with the latest shots given in March last year.
Dr Thongchai said that 23 people in the country were reported to have experienced blood clots after receiving the AstraZeneca vaccination, with seven confirmed to have been linked to the vaccine. Two of the confirmed cases died, he said.
“Covid is no longer classified as a dangerous communicable disease, so current inoculation efforts must use vaccines that have undergone impact studies and are officially registered,” he said. Only the mRNA vaccine from Pfizer is currently registered for use in the country, he said.
Adverse effects from mRNA vaccines were mostly reported in North America, he added.