Every flu season is predictably unpredictable, with the current one a prime example thanks to an unexpected virus strain becoming predominant early on.
According to the CDC’s most recent FluView data, the agency’s weekly influenza surveillance report, the influenza B/Victoria strain is currently the most common flu strain in the U.S. An early edition of the CDC’s Morbidity and Mortality Weekly Report noted that the last time influenza B viruses were the predominant U.S. flu virus was the 1992-1993 flu season. They added that B/Victoria viruses account for 60% of circulating U.S. viruses this year, whereas in the last 3 years, this strain accounted for less than 10% of circulating viruses.
Influenza B viruses circulate every year, but they are generally seen more often at the end of the flu season, towards the spring.
“This year [influenza B] has not only dominated, but it arrived early. That is unique, but every year we have something unique that happens with flu,” virologist Pedro Piedra, MD, of Baylor College of Medicine in Houston, told MedPage Today.
He offered several epidemiological reasons to explain why influenza B has been predominant this year.
“It’s dependent on temperature, humidity, and the proportion of individuals who are not immune, but susceptible. For whatever reason, the environment and the community were more susceptible to B/Victoria this year,” Piedra said.
CDC’s most recent data found that flu activity is “widespread” in 46 states and Puerto Rico. Yet this 2019-2020 flu season bears little resemblance to the last high severity flu season in 2017-2018, with its record number of influenza-related hospitalizations and deaths. CDC reported that 5.8% of deaths during the week of Jan. 4, 2020 were related to pneumonia and influenza, which is below the epidemic threshold of 6.9% for the comparable week. Cumulative influenza-associated hospitalizations were 14.6 per 100,000, which the agency described as “similar to what has been seen during recent previous influenza seasons this time of year.”
“For elderly adults, the virus that normally has the greatest impact in terms of hospitalization and mortality is [influenza A] H3N2, so … we won’t see the impact in terms of hospitalization or death,” Piedra said. “But influenza B tends to hit pediatric patients and young adults disproportionately harder.”
Aaron Glatt, MD, spokesperson for the Infectious Diseases Society of America told MedPage Today that “previous exposures [to flu] help make the immune system as good as possible.”
“Less prior exposures may make it more virulent,” said Glatt, who is chairman of medicine at Mount Sinai South Nassau in Oceanside, New York. “There are more children who have died [this year] at the same point in time as previous years.”
So far, 32 children have died from flu-related illness in the 2019-2020 flu season, including 21 deaths associated with influenza B viruses and 11 with influenza A viruses. A/H1N1(pdm09) is the other main circulating strain, and Piedra said that “older folks have a lot of immunity against that type of virus.”
Glatt also offered up some conjecture about the higher attack rates for influenza B, noting that the trivalent influenza vaccine only has one B strain, whereas the quadrivalent vaccine has two B strains.
Piedra said that the issue will be how well the vaccine virus responds to the type of virus circulating, noting that influenza vaccine efficacy against B viruses is typically around 60%.
“It’s not a perfect match, but vaccines are still the best bet to provide protection,” he said. “Almost every year, one of the viruses is not perfectly matched.”
While CDC’s most recent data said the portion of positive tests for influenza at clinical laboratories decreased, Glatt said that clinicians should still urge their patients, especially pediatric patients and their families, to get the flu shot, which is tied to lower flu duration and severity.
“People say ‘Oh, this week, there’s been a slight decline’ — they shouldn’t be lulled into a false sense of security. Flu waxes and wanes; people shouldn’t think it’s over,” he noted.
Source: MedicalNewsToday.com
