Sanofi Pasteur has manufactured about 70 million doses of its flu vaccine for the upcoming 2018-19 US flu season.

The cyclical vaccine production process is bookended by the newest strain’s distribution in the late summer, and the selection of strains by the World Health Organization (WHO) and US Centers for Disease Control and Prevention (CDC) on March 1. Regardless the trend of previous years’ strains or mutations, manufacturers are working on limited clinical insight as to how best manage the upcoming year’s flu.

David Greenberg, MD, associate vice president and regional medical head of North America for Sanofi, told MD Magazine it’s difficult enough to predict whether the flu will spread more in October, or later in the season like January—let alone predicting which strain will dominate in cases.