HOUSTON National health experts are worried about what they call a twindemic this season. Health experts said they are concerned that flu season and COVID-19 pandemic could collide this year. Infectious Disease Specialist William Schaffner, MD, recommends getting the vaccine between mid-September and early November. Doctors added that even if you do get sick despite getting the vaccine it will be less severe. Experts say that this years flu vaccine will protect against four strains of the flu instead of three as in previous years.