The flu season officially kicked off on Oct. 1, and while most states are reporting only sporadic flu activity, experts say now is the best time of year to get vaccinated to ensure the highest level of protection.
Infographic: How to avoid the flu when you fly
The states where flu season has already started
According to Friday's Weekly Influenza Surveillance Report from CDC, which includes data as of Oct. 19, Maryland is the only state currently experiencing widespread flu activity, and Rhode Island is the only state experiencing no flu activity.
Meanwhile, Arizona, California, Kentucky, Mississippi, South Carolina, and Tennessee are experiencing local flu activity, and the remaining reporting states are experiencing sporadic flu activity. CDC's report said Nevada was the only state that did not report flu activity data to the agency.
CDC has confirmed 126 positive respiratory specimens for influenza A and 269 positive specimens for influenza B in the week ending in Oct. 19. Since Sept. 29, CDC said it has confirmed 1,613 positive specimens.
CDC data show the percentage of outpatient visits for flu-like illness was 1.7% for the week ending in Oct. 19, below the national baseline of 2.4%. The agency also reported the first two pediatric deaths of the 2019-2020 flu season in the week ending in Oct. 19.
Why now is the best time to get your flu shot, according to research
While it's still early in the flu season, experts recommend that now is the best time to get your flu shot.
Research has found that the flu vaccine becomes less effective over time, so if you immunize too early, your protection may have waned by the flu season's usual peak in January and February. On the other hand, if you wait too long to get vaccinated, you may not be protected as flu season ramps up.
One 2017 study from CDC looked at data from four recent flu seasons in the United States and found that the flu shot was most effective immediately following vaccination and became less effective at varying rates depending on the type of flu virus. For influenza A (H3N2) and B, the vaccine's effectiveness dropped 7% per month following vaccination, while for influenza A (H1N1), the vaccine's effectiveness dropped 6 to 11% each month.
Another 2018 study, which looked at seven past flu seasons, found similar results. "Across all seasons, every additional 28 days between vaccination and influenza testing was associated with, approximately, a 16% increase in the odds of testing positive for any influenza," the authors of the study wrote.
In a 2018 editorial published in the journal Clinical Infectious Diseases, two researchers wrote, "Though the root causes of intraseasonal waning immunity are up for debate ... the evidence towards its existence is becoming increasingly difficult to ignore."
For now, the authors wrote, "The prospect that a vaccine administered in August loses effectiveness in as little as three or four months is troubling," because "the millions of people who are vaccinated in August and September may be at risk by January or February, historically when influenza season peaks."
If you already received your flu vaccine in August or September, there may not be much that can be done for this year: CDC says there's no benefit to vaccinating twice within one flu season (Bean, Becker's Clinical Leadership & Infection Control, 10/25; Belluz, Vox, 10/25; CDC Weekly U.S. Influenza Surveillance Report, 10/19; CDC Flu Season FAQ).