Researchers from UCLA reported in the journal Science that exposure to flu viruses during childhood gives people partial protection for the rest of their lives against related viruses.

The scientists looked at data from two types of bird flu and found evidence of unrecognized human immunity against several viruses that circulate in animals but have not previously circulated in humans. When the child was born determined immunity.

The two viruses that were studied — H5N1 and H7N9 — have affected hundreds of people in Asia.

Scientists found that children and young adults are more likely to be infected with H5N1, while H7N9 primarily infects older adults.

The year 1968 appeared to be a pivotal birth year to determine which type of flu virus a person is susceptible to. That was the year of the Hong Kong flu pandemic, which replaced the viruses that had dominated flu seasons for the previous 50 years.

 People born during and since 1968 are more likely to have protection against H7N9, which is more closely related to the 1968 Hong Kong virus than to viruses that spread before 1968.

This study shows that flu exposure may result in lifelong immunity to future infections. Vaccination does not provide lifelong immunity.

In other words, immunization against viruses and bacteria may cause future problems, because that type of immunity can wear off.

That means, especially, that immunizing children against the flu may cause consequences in later years if the child is continually susceptible to infection from influenza viruses.