A new study suggests that a Sinopharm vaccine offers poor protection from the coronavirus disease 2019 (Covid-19) among the elderly, raising 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 percent under 50 years old developed protective antibodies. But the percentage declined with age, and 50 percent 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 of the vaccine developed by Sinopharm’s Beijing Institute of Biological Products.

“This is very, very worrying that these people, who are high-risk, have a poor antibody response,” said Jin Dong-yan, a Hong Kong University virologist who was not affiliated with the study.

Antibody levels are not a direct measure of how protected a person is from Covid-19, but there is growing evidence that they are a good proxy. One expert cautioned that the choice of test kits could have limited the accuracy of the measurements.

Still, the study’s findings have value and are the first public, scientific attempt to analyze the effect of the Sinopharm vaccine in the elderly, said Wang Chenguang, a former professor at Peking Union Medical College and an immunology expert.

China’s National Health Commission declined to comment on the study, saying it would only respond to studies by governments or major research institutions.

This is not the first time questions have been raised about the efficacy of the vaccine, which was given the greenlight by the World Health Organization (WHO) in May and is being used in more than 50 countries, many of which seized upon it when other vaccines were tough to come by.

A spokesperson for the WHO said Wednesday that its experts “are aware of the study and continue to look at all available evidence.” The agency’s advisers raised questions months ago about whether it provided protection in people 60 and over, but when it was OK’d a WHO expert said that there was no reason to think it would work differently in the elderly.

The vaccine is one of two similar shots developed by Sinopharm. The state-owned Chinese company’s research showed that almost all the participants in final-stage clinical trials were under 60 — and its own researchers said there was insufficient evidence to say whether the vaccines work in the elderly. Overall, the Beijing Institute vaccine was found to be 78-percent effective.