Atlanta—Influenza vaccine effectiveness during the most recent flu season was similar to previous years when the vaccine viruses also were a good match to circulating viruses, according to an article in the Morbidity & Mortality Weekly Report.
The CDC reports that final vaccine effectiveness estimates of 34% against illness caused by influenza A(H3N2) viruses and 56% against illness caused by influenza B viruses were in line with expectations. Even the lower effectiveness observed against well-matched A(H3N2) viruses was no surprise.
“Evidence of reduced protection against A(H3N2) viruses, even when vaccine viruses and circulating viruses are well matched, has been observed since the 2011–2012 season,” the CDC researchers point out. “In general, vaccination with inactivated influenza vaccine has offered better protection against influenza A(H1N1) and influenza B viruses.”
The public health officials add that, even during seasons when vaccine effectiveness is reduced, widespread immunization reduces the risk of severe outcomes such as hospitalization and death. As an example, they cite the 2012–2013 season: Despite an estimated vaccination effectiveness against A(H3N2)-related illness of 39%, vaccination was judged to have averted an estimated 5.6 million illnesses, 2.7 million medical visits, 61,500 hospitalizations, and 1,800 deaths.
In general, according to the article, the 2016–2017 influenza season was most notable for the predominant circulation of influenza A(H3N2) viruses. The report states that, nationally, influenza activity peaked in mid-February, with influenza A(H3N2) viruses predominant early in the season through the week ending March 25 (week 12). The next week, influenza B viruses became predominant and remained that way through the end of May.
Interestingly, influenza activity peaked a month earlier in the western United States than in the rest of the country, according to the research.
Study authors note that hospitalization and mortality rates late last year and early this year were similar to other flu seasons when influenza A(H3N2) viruses predominated.
“Previous influenza A(H3N2)–predominant seasons have been associated with increased hospitalizations and deaths compared with seasons that were not influenza A(H3N2)–predominant, especially among children aged <5 years and adults aged ≥65 years,” they write.
In general, the CDC says the vaccine was a good match, and no antiviral resistance to oseltamivir, zanamivir, or peramivir was identified among tested influenza viruses from the 2016–2017 season.
While summer influenza activity in the U.S. tends to be low, public health officials still urge clinicians to remain vigilant. They point out that outbreaks have occurred during the summer months but can be difficult to differentiate from summer respiratory illnesses. The CDC urges testing for seasonal influenza viruses and monitoring for novel influenza A virus infections all year.
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