If you think your child’s annual flu vaccine only helps for one season, think again. New findings show years of season-specific flu vaccines help to develop  cumulative, broader protection against new flu strains.

Researchers at McMaster University in Canada also found the longer-term protection extends to flu strains that are capable of causing pandemics. To discover that, they spent more than three years studying immune responses in kids between the ages of 6 months and 17 years.

During the study, the researchers compared two types of vaccines — the conventional injection and a nasal spray that takes hold in the upper respiratory tract. Bother were found to be equally effective in generating broadly protective antibodies.

After three vaccination seasons, virus neutralization increased more than 2½ times in younger children. The researchers said that supports the idea that seasonal flu vaccines can prompt a sustained antiviral response.

That is especially good news for younger children — but less so for teenagers and adults. The researchers noted that as children get older, they were less able to produce broadly protective antibodies due to repeated viral exposure and other factors. Adults are completely out of luck: They get no long-term protection from repeated flu vaccines.

The findings are an important step toward developing a universal flu vaccine for children — an accomplishment that has proven to be elusive. For now, seasonal flu vaccines are chosen each year based on surveillance data showing which viruses are circulating and making people sick.

But for younger kids, repeated flu vaccines may be a boon: A little pain but a lot of protective gains.