The long-feared bird flu pandemic may not be as devastating as medical professionals have been warning for years. Researchers in Rotterdam discovered that antibodies against the regular flu, either from getting the flu or the flu shot, work surprisingly well against the H5N1 virus, the Volkskrant reports.

So people who have had the common flu or gone to get their flu shot also have immunity to bird flu to a certain extent. “Based on this, you would expect that they could become infected and pass the virus on to other people, but not necessarily become very ill,” virologist Rory de Vries of Erasmus MC told the newspaper. The researchers will publish their findings in The Lancet Microbe on Tuesday.

The results are based on research of blood samples, not of people with bird flu. So more research is needed. But the researchers are optimistic. “What surprised us is that there is already quite a bit of immunity present in the blood,” De Vries said. “My gut tells me that the disease burden of bird flu might actually turn out to be less severe than expected.”

The researchers tested the blood of 107 hospital employees and found antibodies in virtually everyone that bind to the bird flu virus in one way or another. Most of these antibodies attach to the “stem” of the bird flu virus spikes, which resemble the human flu. That is not enough to prevent infection or transmission of the virus, but it is enough to alert other immune cells.

The Rotterdam researchers also found that T-cells, which attack virus particles, respond very quickly to the bird flu virus. “The only thing missing is the crucial neutralizing antibodies,” De Vries said. These antibodies prevent infection.

Medical professionals have worried for years that the “highly pathogenic” H5N1 would mutate to infect humans and cause devastation on a level similar to the COVID-19 pandemic.

Worldwide, about 1,000 people have now been infected with the bird flu virus. Roughly 30 to 40 percent of them died, a terrifyingly high proportion. But according to the Rotterdam researchers, their data suggests that many more people have been infected with H5N1 without even noticing. “It is possible that we are missing a great many mild cases.”

“This is something that makes me happy,” said Debbie van Baarle, a professor of immunology at the University Medical Center Groningen. She was not involved in the study, but is familiar with the results. “In preparing for pandemics, it is important to be able to identify exactly who the most vulnerable groups are. And this research provides a good starting point for that,” Van Baarle told the Volkskrant.

She pointed out that the Rotterdam figures suggest that older people’s immune cells respond better to bird flu, presumably because they have been exposed to more flu variants. Van Baarle calls that “hopeful,” given that older people generally get sicker from the flu.