There is a much higher risk of brain blood clots from Covid-19 infection than there is from vaccines against the disease, British researchers said on April 15, after the rollout of inoculations was disrupted by reports of rare clots.

A U.S. health advisory panel on April 14 is set to review six reported cases of rare blood clots in women who received Johnson & Johnson’s Covid-19 vaccine one day after federal regulators paused the use of the shot to assess the issue.

U.S. federal health agencies on April 13 recommended pausing the use of Johnson & Johnson’s Covid-19 vaccine after six women under 50 given the shot developed rare blood clots, dealing a fresh setback to efforts to tackle the pandemic.

Europe’s drug regulator is reviewing reports of rare blood clots in four people who received Johnson & Johnson’s Covid-19 vaccine and expanded its probe into AstraZeneca’s shot to include reports of a bleeding condition.

The European Medicines Agency (EMA) said on April 7 it had found a possible link between AstraZeneca’s coronavirus vaccine and reports of very rare cases of blood clots in people who had received the shot.

The World Health Organization expects there will be no reason to change its assessment that the benefits of the AstraZeneca vaccine against Covid-19 outweigh any risks, a regulatory official told a news conference on April 6.

Europe’s medicines regulator said on March 31 the EMA had not identified any risk factors such as age, sex or a previous history of blood clotting disorders, for clotting cases reported after inoculation with AstraZeneca’s Covid-19 vaccine.

Fewer Europeans trust the AstraZeneca Covid-19 vaccine after several countries reported side effects, such as blood clots, an opinion poll by YouGov showed on March 22, even though scientific studies have found it is safe and effective.

Scientists are exploring several possibilities that might explain at least 18 reports of extremely rare blood clots in the brain that occurred in individuals in the days and weeks after receiving the AstraZeneca COVID-19 vaccine.

AstraZeneca Plc said on March 14 a review of safety data of people vaccinated with the company’s Covid-19 vaccine has shown no evidence of an increased risk of blood clots.