The US could be heading for one of its worst flu seasons in years

As summer turns to fall, the medical community is ramping up for their annual fight against the flu.

One Wall Street firm thinks this flu season could be a bad one.

William Quirk and Alexander Nowak at Piper Jaffray use the number of flu cases from Australia and New Zealand to forecast the US season, and they say the numbers from the southern hemisphere point to a massive increase in sicknesses this year.

'The Southern hemisphere just went through one of the worst flu seasons on record (despite the vaccine targeting the correct strains)," wrote Quirk and Nowak in a note to clients. "Using the New Zealand influenza season (reinforced with Australia) as our basis (influenza-like illness cases +45% yoy), our analysis suggests a 31.9% rise in influenza-like illnesses (ILI) season/season for the United States."

This would mean that their average prediction for US cases is 943,356, which is the worst since the H1N1 pandemic in 2009. Last year there were 715,031 reported ILIs, and 2009 was the only year in a decade to crack 750,000.

A particular concern is that the flu vaccine, which must be reconfigured every year as the illness evolves, is unable to stem the tide.

"This year, Australian ILIs surpassed recorded levels set during the 2009 H1N1 pandemic, while New Zealand 's ILIs are inline with 2009," said Quirk and Nowak. "This is despite the vaccine targeting the predominate strains (H3N2 A/Switzerland, B/Yamagata lineage and B/Victoria lineage), suggesting the flu is more virulent this year."

Strains that hit the southern hemisphere have migrated to the US, so the experience could prove similar.

The analysts performed regression analysis of New Zealand and US flu cases, and found a notably close correlation.

While there's a large margin for error, but the low-end of their prediction range is still as high as it was last year. At the high end, the US could experience close to 1.2 million cases of the flu this year.