Interim estimates from the Centers for Disease Control and Prevention (CDC) put influenza vaccine effectiveness at just 36% overall in the United States for the 2017-2018 season, researchers report in an article published today in Morbidity and Mortality Weekly Report.
“Even with current vaccine effectiveness [VE] estimates, vaccination will still prevent influenza illness, including thousands of hospitalizations and deaths. Persons aged ≥6 months who have not yet been vaccinated this season should be vaccinated.” write Brendan Flannery, PhD, from the agency’s Influenza Division, National Center for Immunization and Respiratory Diseases, and colleagues.
Because the vaccine is not completely protective, the CDC researchers also stressed the need for early consideration of antivirals, even in vaccinated patients, and for considering influenza as a possible diagnosis for all patients with acute respiratory illness.
“These interim VE estimates underscore the need for influenza antiviral treatment for any patient with suspected or confirmed influenza who is hospitalized, has severe or progressive illness, or is at high risk for complications from influenza, regardless of vaccination status or results of rapid, point-of-care influenza diagnostic tests. CDC recommends antiviral medications as an adjunct to vaccination, and their potential public health benefit is increased in the context of low VE.”
According to the CDC, vaccination reduced illness caused by influenza A(H3N2), the predominant virus, by 25% among patients of all ages and by 59% in children 6 months through 8 years of age.
The vaccine was 67% effective against the less common influenza A(H1N1)pdm09 viruses and 42% effective against influenza B viruses.
When stratified by patient age, irrespective of viral strain, VE was 59% for ages 6 months to 8 years; 33% for ages 18 to 49 years; and not significantly effective for ages 9 to 17 years, 50 to 64 years, or 65 years or older.
Influenza hospitalizations, influenza-like-illness, and mortality rates remain at historically high levels across the country. CDC guidance repeatedly emphasizes that it is not too late to get a flu shot and that even an imperfectly protective vaccine can save lives.
The researchers used methods developed by the US Flu VE Network to enroll 4562 patients at five study sites who were older than 6 months of age and were seeking outpatient medical care for an acute respiratory illness with cough and onset within the previous 7 days. Patients were enrolled between November 2, 2017, and February 3, 2018. Patients or their proxies were interviewed for information, including vaccination status, and nasal and oropharyngeal swabs (or nasal swabs alone for children younger than 2 years) were analyzed by using the CDC’s real-time reverse transcription polymerase-chain reaction protocol for detection and identification of influenza viruses.
Among the 4562 patients tested, 38% tested positive for influenza virus, of which 81% were influenza A viruses and 19% were influenza B viruses. Among the influenza A viruses, 85% were A(H3N2) and 16% were A(H1N1)pdm09. Among the B viruses, 98% were from the B/Yamagata lineage.
The researchers found that 53% of influenza-negative patients and 43% of influenza-positive patients had received the 2017-2018 seasonal influenza vaccine.
The authors emphasize that a limitation of this analysis is its focus on prevention of outpatient medical visits rather than hospitalizations or deaths. Low efficacy for seasonal influenza vaccines remains a worldwide public health concern, but Flannery and colleagues point out that during the 2014-2015 season, VE was less than 20% (substantially lower than for the current vaccine), but was still estimated to have prevented as many as 144,000 influenza-associated hospitalizations and between 3400 and 4000 influenza-associated deaths.
The authors have disclosed no relevant financial relationships.
MMWR Morb Mortal Wkly Rep. 2018;67:180-185. Full text
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