ST. LOUIS (KMOX) - Health officials say this year’s flu shot was not as effective as other years’.

“We change the flu vaccine on an annual basis to keep up with the changing viruses and it varies in its effectiveness from year to year,” infectious diseases specialist William Schaffner explains.

That may be part of the reason why Missouri has seen a 212 percent increase in flu cases this year. There have been nearly 30,000 flu cases in Missouri this season compared to an average of about 10,000 the past five years.

Schaffner says the flu shot was especially ineffective against seniors.

“This year, against the dominant strain, it was only about nine percent effective in seniors but if you look at all the strains together, it was 27 percent effective. That’s not something to write home about but still preventing hundreds of thousands of infections and hospitalizations.”

Schaffner says specialists will work again later this year to come up with better protection.