Vaccines that protect against multiple influenza strains are possible, according to two studies, raising hopes for more persistent immunization than current products can provide.

Using different approaches, the research groups showed that a relatively unchanging part of the virus can be used to elicit an immune response that blocks infection by several different types of flu.

But outside experts cautioned that the studies were conducted in the lab and in experimental animals and it will likely be some time before a “universal” vaccine is available for human use.