Flu Vaccine Just 25% Effective Against Dominant Strain This Year

Marcia Frellick and Janis Kelly

February 15, 2018

Anne Schuchat, MD, acting director for the Centers for Disease Control and Prevention (CDC), said in a national briefing today that flu activity across the nation remains high and is likely to continue "for several more weeks."

"Nationwide, levels of influenzalike illness...are currently around where we observed during the peak of the H1N1 2009 pandemic. This doesn't mean that we're having a pandemic right now, just that levels of influenzalike illness are around as high as we saw in 2009," she said.

Hospitalization rates are at a decade high, she said, for cases typically seen at this time of year.

A vaccination effectiveness (VE) study published today in Morbidity and Mortality Weekly Report shows that flu vaccines were 36% effective overall against influenza A and B, which means a vaccinated person reduced his or her risk of getting sick with the flu by about one third.

However, the data show that VE against the season's dominant strain, the H3N2 strain, was lower, at 25%, Schuchat said.

One reason this season has been so challenging is that the A(H3N2) viruses have been linked to more severe illnesses. In recent weeks, more B viruses and H1N1 viruses have been circulating, for which the vaccine is somewhat more effective. Specifically, the vaccine is 67% effective against the A(H1N1)pdm09 viruses and 42% effective against influenza B viruses.

Reports show that 63 children have died of flu so far this season, Schuchat said, adding, "Unfortunately, we're expecting to report more pediatric deaths in the near future." She noted that of those children who died, 3 of 4 had not received a flu shot.

Joining Schuchat for the briefing were US Health and Human Services (HHS) Secretary Alex Azar; Surgeon General Jerome Adams, MD, MPH; US Food and Drug Administration (FDA) Commissioner Scott Gottlieb, MD; National Institute of Allergy and Infectious Diseases Director Anthony Fauci, MD; and Assistant Secretary for Preparedness and Response Robert Kadlec, MD.

Azar emphasized in the briefing that all the experts on the briefing panel agree that getting a flu shot, even this late in the season, will help protect Americans.

Surgeon General Adams said, "The flu vaccination is safe, and it is still your best defense. Getting vaccinated does not mean that you can't or won't get sick, but it can reduce the duration and severity of illness and limit spread to others."

Work will begin soon on developing next season's vaccine.

According to a statement released by the FDA today, in 2 weeks, experts will meet to select strains for next season's flu vaccines, because the vaccines take months to produce.

Asked whether in hindsight different steps could have been taken that would have resulted in a milder flu season, Schuchat said the strains that were predicted to circulate were the correct ones, but "the problem is the vaccine's not working as well as we had hoped in general.

"There are questions about whether the virus that's grown up in eggs to produce most vaccines is mutating or changing in ways that evade the vaccine," she continued. "That's an issue we're looking at very intensely across the department and with the private sector to understand whether there are better ways to make sure we're producing effective vaccines."

Vaccine Efficacy Study

The authors of the MMWR article, led by Brendan Flannery, PhD, from the FDA's Influenza Division, National Center for Immunization and Respiratory Diseases, stress the need for early consideration of the use 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."

When stratified by patient age, irrespective of viral strain, VE was 59% for persons aged 6 months to 8 years; 33% for those aged 18 to 49 years; and not significantly effective for those aged 9 to 17 years, 50 to 64 years, or 65 years or older.

The researchers used methods developed by the US Flu VE Network to enroll 4562 patients at five study sites. Patients were older than 6 months and were seeking outpatient medical care for an acute respiratory illness with cough and onset within the previous 7 days. They were enrolled between November 2, 2017, and February 3, 2018. Patients or their proxies were interviewed for information, including vaccination status. Nasal and oropharyngeal swabs (or nasal swabs alone for children younger than 2 years) were analyzed in accordance with 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; 81% of cases 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.

MMWR Morb Mortal Wkly Rep. 2018:67;180-185. Full text

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