AMSTERDAM - An unexpected increase in the number of people exhibiting different types of blood clots after being given the first dose of the AstraZeneca Covid-19 vaccine warrants more research, Dutch side effects center Lareb said. Aside from the established link between the vaccine and a combination of blood clots with a low platelet count, the center also received more case reports than expected for other clotting, like deep vein thrombosis and pulmonary embolism.

In data gathered from four million vaccinations, the number of side effect reports amongst people vaccinated with AstraZeneca was one and a half times higher than expected. Lareb said this was not enough to draw a link between the vaccine and certain blood clot cases as a higher rate of those vaccinated early are people more likely to be at higher risk of a clot incident. That includes those of advanced age, and people who are obese.

After about a million shots of the AstraZeneca vaccine had been given in the Netherlands, 150 blood clot incidents were reported in total, of which 136 were considered serious. Four resulted in death through April 14. The patients had a stroke 42 times, there were 38 cases of pulmonary embolism, and 24 of deep vein thrombosis. There were 63 other trombo-embolism events. Those cases include nine of the people who had a combination of a blood clot with a low platelet count, already considered a rare side effect of the vaccine.

In the Lareb study into the development of thrombosis without a low platelet count, the organization received 399 reports of thrombosis those vaccinated. The other 249 cases were identified amongst the three million people who received the Pfizer/BioNTech or the Moderna vaccine. In both of those groups, the number of blood clot cases reported were less than expected.

The side effects research centers said it is too soon to draw any firm conclusions. Many uncertainties such as how often these forms of thrombosis occur in the general population still remain. It also said that the number of reports increased after the first news stories came out regarding concerns over the AstraZeneca vaccine. That increased awareness may be partly responsible for the increase in reports filed with Lareb.

AstraZeneca vaccines for people under 60 were stopped last month due to a possible link between the vaccine and thrombosis with a low platelet count. This reaction to the vaccine was found in twelve people in total in the Netherlands, all women between the ages of 23 and 65, one of whom died.