Whooping cough rebounds to pre-pandemic levels

• Infants under 1 year old with still-developing immune systems are at greatest risk of serious illness from infections and also have the highest reported rate by age group.

Current situation: There have been at least 15,661 cases in the U.S. this year as of Sept. 21, per the CDC.

• Pennsylvania had the highest number, with 2,087, followed by New York with 1,781.
• Mid-Atlantic states had the most by region, with just under 4,000 cases.
• Last year, about 84% of infants worldwide (108 million) received three doses of the diphtheria-tetanus-pertussis vaccine, according to the World Health Organization.

But lingering vaccine hesitancy from the pandemic has left more parents opting out of shots using non-medical religious exemptions.

• Public health officials also are concerned that circulating strains of pertussis are adapting to evade vaccines, and that immunity in previously vaccinated individuals is rapidly waning.
• Advisers to the Food and Drug Administration met this month to discuss new, more durable vaccines, including nasal vaccines that could boost immunity through mucous membranes.

For now, boosters are recommended roughly once every 10 years, starting in middle school.

• It's the teens whose immunity against whooping cough has waned who are driving the outbreaks in many states, experts told NBC News.