About half of this season’s deaths were in otherwise healthy children. They ranged in age from 8 weeks to 17 years. Of those for whom a flu shot is recommended, less than one-fourth of the children who died had been fully vaccinated. That was about the same proportion as in past winter flu seasons.
The number of pediatric flu deaths “is a record number since we’ve been keeping track, outside of the pandemic,” said Daniel Jernigan, who heads the CDC’s influenza division. And the number is considered an undercount because it includes only lab-confirmed cases that are listed on death certificates and then reported to the CDC. It could go even higher because of reporting delays.
“It’s probably half of what actually occurs, because there are times when children die even before they get to the emergency room,” Jernigan said in an interview. More than 40 percent of this season's young victims died before they were hospitalized.
States are not required to report individual seasonal flu cases or adults' deaths. The CDC uses mathematical modeling to estimate total flu-related deaths. Because flu seasons vary in length and severity, the agency estimates that flu-related deaths in the United States ranged from a low of 12,000 (during 2011-2012) to a high of 56,000 (during 2012-2013).
Data released this week by CDC give the fullest picture so far of a season that was notable for the volume and intensity of cases, which overwhelmed hospitals and led Alabama to declare a state of emergency. Some hospitals were forced to pitch tents outside emergency rooms. Other facilities had bed shortages that kept ambulances idling outside.
This season’s predominant strain was H3N2, the most dreaded flu strain. It’s associated with more complications, hospitalizations and deaths, especially among children, people older than 65 and those with certain chronic conditions.
But even though officials knew it was likely to be a harsh season, they weren’t expecting virtually the entire country to be slammed at the same time and across all age groups. In past years, the flu more commonly appeared in different parts of the country at different times.
Flu seasons typically last between 16 to 20 weeks. The 2017-2018 season was 19 weeks. Flu activity began ramping up in November, reached high levels in January and February, and stayed elevated through March.
Last year, the CDC developed a new way to classify flu season severity based on three indicators: the percentage of people going to doctors’ office for fever, cough and other flulike symptoms; rate of hospitalizations; and percentage of deaths from pneumonia or flu. Using that index, officials analyzed each season going back to 2003-2004.
Their conclusion: This season was the first — and only — to be classified as “high severity” for all age groups.
“It wasn’t just a bad year for some people. It was bad across the population,” Jernigan said. “It was happening to everyone, and it seemed to be happening everywhere.”
A CDC flu summary released Thursday noted other indicators of the season's severity:
• At its peak in early February, the percentage of people going to doctors’ offices with fever, cough and aches was the third-highest since the 1997-1998 season, when the CDC surveillance system was put in place.
• The percent of deaths caused by pneumonia or flu reached a peak of nearly 11 percent, or 1 out of 9 people, the highest since the 2014-2015 season, when the same strain was dominant.
• The total hospitalization rate for children and adults was the highest since 2005.
It’s not common for two consecutive flu seasons to be dominated by H3N2, although that's what happened in 2016-2017 and 2017-2018. No one knows how the next season will shape up, or whether the same virulent strain will dominate, but “sometimes we’ve had two or three in a row,” Jernigan noted.
This season’s vaccine offered limited protection against the flu viruses that swept the country, with an overall effectiveness of 36 percent falling to 25 percent against the H3N2 strain, according to preliminary data released in February. But in children younger than 9, it offered much greater protection, reducing by more than half the risk of becoming so sick that they needed to see a doctor.
Jernigan said the coming season's vaccine production process includes a strain that is expected to perform better against the H3N2 strain.