People who got vaccinated against the H1N1 swine flu virus may also be protected against the strain of influenza that killed 50 million to 100 million people in 1918, researchers reported on Tuesday.

Tests on mice showed the vaccine for the still-circulating strain of H1N1 protected against the older virus, a distant cousin also called H1N1, the team at Mount Sinai School of Medicine in New York said.

No one fears the 1918 flu will come back naturally but there are worries that someone might try to resurrect or recreate it for a biological attack. The study, published in Nature Communications, also shows the vaccine has cross-protection against related flu strains.

The old virus was reconstructed by scientists using samples dug up from the frozen bodies of victims in Alaska and using genetic sequences from preserved samples.

While the reconstruction of the formerly extinct Spanish influenza virus was important in helping study other pandemic viruses, it raised some concerns about an accidental lab release or its use as a bioterrorist agent, said Adolfo Garcia-Sastre, who led the study.

Our research shows that the 2009 H1N1 influenza vaccine protects against the Spanish influenza virus, an important breakthrough in preventing another devastating pandemic like 1918.

The researchers gave mice either the 2009 H1N1 influenza vaccine, the seasonal H3N2 flu vaccine or no vaccine at all.

After 21 days, the mice were exposed to a deadly dose of the 1918 Spanish influenza virus.

The mice vaccinated against H1N1 swine flu lived. About 80 percent of mice vaccinated with a seasonal flu vaccine designed to protect against a 2007 strain of H1N1 lived when infected with the 1918 flu, the study found.

All of the mice that were given the H3N2 vaccine or not vaccinated died when infected with the 1918 flu.

Current flu vaccines are a mixture of the three most common strains of virus circulating. The formula is changed slightly every year as flu viruses mutate. For instance, this coming flu season the H1N1 component will be replaced with the pandemic swine flu strain.

There is evidence people who get seasonal flu vaccines are protected to some degree against future flu strains. This study showed they also can be protected against a past strain.

It also suggested 2009 H1N1 is more closely related to the old pandemic strain than to more modern H1N1 strains, which have had decades to mutate and change.

The swine flu strain came from pigs and is a mixture of several influenza viruses, genetic tests have shown.

Considering the millions of people who have already been vaccinated against 2009 H1N1 influenza, cross-protection against the 1918 influenza virus may be widespread. Our research indicates that people who were exposed to the virus may also be protected, Garcia-Sastre said.

Dozens of companies make flu vaccines, including Sanofi Aventis, AstraZeneca, GlaxoSmithKline, Baxter and others.

(Reporting by Maggie Fox; Editing by Julie Steenhuysen and John O'Callaghan)

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