If you’re worried about getting the flu, now is the time to get your shot. It takes between two and six weeks after receiving a flu shot for its protective effect to take hold. If you’ve had the vaccine before that point, it doesn’t provide protection against this year’s strain of influenza virus.
And because this year’s vaccine isn’t as effective against H3N2 viruses, some health care professionals are advising their patients to delay getting vaccinated until February or March 2019. That advice will be based on which strains of influenza virus are most prevalent in the U.S., says Dr. Daniel Hanley with Northwell Health at Stony Brook University Medical Center in New York State