COVID-19: WHO experts want more AstraZeneca clot data from outside Europe

The World Health Organization's vaccine advisors called on Thursday for more data on the incidence of blood clots in people who received the AstraZeneca Covid-19 vaccine outside Europe.

The WHO's Strategic Advisory Group of Experts (SAGE) on Immunisation updated their guidance on the use of AstraZeneca's coronavirus jab, rewriting the section on precautions in light of data from Europe on clotting.

"WHO continues to support the conclusion that the benefits of these vaccines outweigh the risks," the UN agency stressed in a statement.

On April 7, SAGE said a link between AstraZeneca's Covid-19 jabs and blood clots was plausible but unconfirmed, underlining that reported occurrences were "very rare".

In light of evidence emerging from ongoing vaccination programmes, they have updated their recommendations on the AstraZeneca jab, which is being deployed in 157 territories.

SAGE said most clotting cases were reported in Britain and the European Union, with very few cases noted in other countries.

They said it was unknown whether there was a risk of clotting from the second dose of the vaccine, while recommending that those who suffered blood clots after their first injection should not be given the second dose of the two-shot vaccine.

"A very rare syndrome of blood clotting combined with low platelet counts, described as thrombosis with thrombocytopenia syndrome (TTS), has been reported around four to 20 days following vaccination," the new guidance says.

"A causal relationship between the vaccine and TTS is considered plausible although the biological mechanism for this syndrome is still being investigated.

"Most of these cases were reported from the UK and the EU. There is considerable geographic variation with regards to the reported incidence, with very few cases reported from non-European countries, despite extensive use of the vaccine.

"An estimation of the risk outside Europe needs further data collection and analysis."

The vaccine currently forms the backbone of the Covax scheme, which ensures that poorer countries can access doses, with donors covering the cost.

Covax has so far shipped more than 40.5 million Covid-19 vaccine doses to 118 participating territories.

The SAGE guidance said that data from Britain, dated March 31, suggested the risk of TTS was approximately one case per 250,000 vaccinated adults, while the rate in the EU was estimated at one in 100,000.

It said current data from Europe suggested that the risk might be higher in younger adults compared with older adults.

"No specific risk factors have yet been identified," it said.

Whilst stressing that the benefit of vaccination against Covid-19 "far outweighs" the risks, the assessment may differ between countries.

"Countries should consider their epidemiological situation, individual and population-level risks, availability of other vaccines, and alternate options for risk mitigation," the guidance said.

"The benefit-risk ratio is greatest in older age groups.

"It is currently unknown whether there is a risk of TTS following the second dose."

The safety surveillance and monitoring section was also rewritten,recommending research into serious adverse events including cerebral venous sinus thrombosis and thrombotic events with thrombocytopenia.

It also called for research into the incidence of TTS by region, age and sex.

(With inputs from agencies)