A century after one of history's worst disease outbreaks, scientists are rethinking how to guard against another super-flu like the 1918 global influenza that killed tens of millions.

But researchers hope they're finally closing in on stronger flu shots, ways to boost much-needed protection against ordinary winter influenza and guard against future pandemics, or outbreaks in many countries.

Labs around the United States are hunting for a super-shot that could eliminate the annual fall vaccination in favor of one every five years or 10 years, or maybe, eventually, a childhood immunization that could last for life. Despite 100 years of science, the flu virus too often beats our best defenses because it constantly mutates, or changes.

Among the new strategies: Researchers are studying the cloak that disguises influenza as it sneaks past the immune system and finding some rare targets that stay the same from strain to strain, year to year.

In 1918, there was no flu vaccine. It wouldn't arrive for decades. Today, vaccination is the best protection, said Anthony Fauci, who's in charge of infectious diseases at the National Institutes of Health. But at best, the seasonal vaccine is 60 percent effective. Protection dropped to 19 percent a few years ago, when the vaccine didn't match an evolving virus.

If a never-before-seen flu strain erupts, it takes months to brew a new vaccine. Doses arrived too late for the last, fortunately mild, pandemic in 2009.

Lacking a better option, the nation is "chasing" animal flu strains that might become the next human threat, Fauci said. Today's top concern is a deadly bird flu that has jumped from poultry to more than 1,500 people in China since 2013. Last year it mutated, meaning millions of just-in-case vaccine doses in a U.S. stockpile no longer match.