A new study suggests that a Sinopharm vaccine offers poor protection from COVID-19 among the elderly.

That raises questions for dozens of countries that have given the Chinese company’s shots to their most vulnerable populations.

A survey of blood samples taken from 450 people in Hungary at least two weeks after their second Sinopharm dose found that 90% under 50 years old had developed protective antibodies. But the percentage declined with age, and 50% of those over 80 had none.

The study by two Hungarian researchers was posted online this week but not yet reviewed by other scientists.

Three outside experts said they had no problems with the methodology of the study.

China’s National Health Commission declined to comment on the results of the study.