Problems with this year's flu shot have contributed to a bad season.

Next season’s flu vaccination is being tweaked for broader protection, due to last year’s inconsistent coverage of influenza strains, U.S. health officials announced.

Because millions of doses are needed for each season, the vaccination is determined months in advance to allot time to make them, the U.S. Centers for Disease Control and Prevention said in its June 5 report. Components of the vaccine have been adjusted to accommodate more circulating viruses for the 2015-2016 season, it stated.

One version of the vaccine, quadrivalent flu vaccine, will protect against two influenza A viruses and two influenza B viruses. The new shots will contain two type A viruses, H1N1 and H3N2, and a type B component. These strains are expected to be the main ones of next season, CDC epidemiologist Lynnette Brammer said.

Last year, only 18.6% of shots were effective against H3N2 because nobody expected that strain to predominate and was not worked on until February, Brammer added. In fact, last season was considered a moderately severe flu season. It affected mostly those 65 and older, leading to many hospitalizations and thousands of deaths, CDC experts reported.

The new vaccine will be available in September.