(CNN) - Lasting 21 weeks, the 2018-19 flu season was not only the longest in a decade, it was unusually marked by two separate waves of influenza A sickness, the US Centers for Disease Control and Prevention confirmed Thursday in a new report.
An H1N1 strain dominated during an initial surge between October through mid-February, followed by a groundswell of H3N2 activity beginning in mid-February and lasting through mid-May, the CDC said. The H3N2 strain is known to cause more severe symptoms than H1N1.
The flu virus is spread when a sick person talks, sneezes or coughs. Common symptoms include fever and chills, cough, sore throat, runny or stuffy nose, muscle and body aches, headaches and fatigue. Despite painful symptoms, most people will recover in less than two weeks.
"It was quite an unusual season in that we almost had two seasons within the one," said Richard Webby, a flu scientist on the World Health Organization's influenza vaccine advisory board and a member of St. Jude Children's Research Hospital's Department of Infectious Diseases.
Webby, who was not involved in the CDC report, said that it's not unusual to see a wave of flu A followed by a smaller wave of flu B but that this longer-than-usual season had "two almost similar peaks of activity" from two strains of flu A.
"It's hard to know what to read into that: Is this an anomaly? Or is this something that's changing?" he said, adding that other countries had unusual seasons as well. Australia had nearly year-round activity for its own 2018-19 season, even though typically its flu seasons are much like the US, with a spike in winter, Webby explained.
The 2018-19 American and Australian seasons could be "just a couple of anomalies that we won't see again or won't see again for a number of years," he said. "We're far from being able to make any conclusions there."
Moderate severity overall
Despite its length and unusual pattern, the CDC considers the season to be of only "moderate" severity, both overall and for each age group (children and teens, adults and older adults).
The flu caused between 37.4 million and 42.9 million illnesses in the US during the 2018-19 season; between 531,000 and 647,000 hospitalizations; and between 36,400 and 61,200 deaths, the CDC estimated.
Deaths markedly declined season over season; the 2017-18 flu season, the deadliest in four decades, topped 80,000 deaths, including 180 children. By comparison, 116 laboratory-confirmed deaths in children were caused by flu during the 2018-19 season.
The 2018-19 season also had lower hospitalization rates for adults, though a similar proportion of children appeared in hospital emergency rooms with signs of illness.
"Receiving a seasonal influenza vaccine each year remains the best way to protect against seasonal influenza and its potentially severe consequences," wrote the authors of the CDC report, which does not estimate vaccination rates or overall effectiveness for the entire 2018-19 season.
Next year's vaccine should be better
Dr. William Schaffner, an infectious disease specialist at Vanderbilt University and medical director of the National Foundation for Infectious Diseases, said that "preliminary assessments" of the vaccine showed it to be "substantially better" against the H1N1 strain than the H3N2 strain.
"That was part of the motivation, of course, to change the H3N2 component for the coming year," said Schaffner, who was not involved in the CDC report.
"The flu vaccine has been reconstituted to be a better match, particularly for the H3N2 component," he said. "Given the fact that there were basically two flu seasons last year, it gives you even more reason to get vaccinated."
Webby said that the unusual double spike of the 2018-19 season speaks to the importance of getting a shot.
"The flu shot does contain all of the different viruses that we saw circulating last year," he said. "It's not 100% effective. It doesn't work as we would like in everybody during every season."
Still, the vaccine is the best and "really the only way" to protect yourself and your family from getting the flu or lessening the severity if you do get sick, he said.
He also noted that "there's some suggestion that, over the course of a season, the flu shot may wane a little bit, meaning that by the end of the flu season, the immunity generated by the typical flu shot may not be as effective as it was at the beginning of the season."
This raises some interesting questions about how best to vaccinate people, Webby said. Do we delay getting a flu shot to get better coverage near the end of the season? Or maybe we can think about getting two flu shots per season. Researchers are examining these questions, Webby said, while scientists work on developing a universal flu vaccine: a single shot to protect against all strains of flu for life.
"We can probably argue about whether getting one flu shot that's going to protect you from everything for life is realistic, but we will certainly be getting improved flu vaccines as a result of this push for the universal flu vaccination," said Webby, who believes that "increased duration of immunity following vaccination" will be discovered.
Schaffner said the "substantial influenza season at the present time" in Australia could be "a harbinger for things to come here. So if you need more reason to get vaccinated, that's a good reason."
Finally, he noted that scientists have found no resistance to antiviral medications, like Tamiflu, which requires a prescription. This is something all patients but especially those with compromised immune systems, like cancer patients, and the elderly should take into consideration, he said.
"If you get the flu, call your health care provider," Schaffner said. "Treatment is available. It helps."
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