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Flu Vax Efficacy 25% Against Predominant H3N2 Strain So Far

Flu Vax Efficacy 25% Against Predominant H3N2 Strain So Far

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Action Points

  • Influenza A (H3N2) comprised about 70% of influenza infections this year, and interim vaccine efficacy against this strain was only 25%, according to the CDC.
  • Note that influenza activity began to increase in early November and rose sharply from December through February, with some of the highest levels of influenza-like illness and hospitalization rates in recent years.

Influenza A (H3N2) comprised about 70% of influenza infections this year, and interim vaccine efficacy against this strain was only 25%, CDC researchers said.

Individual vaccine efficacy ranged from 25% (95% CI 13%-36%) against influenza A (H3N2) to 42% (95% CI 25%-56%) against influenza B viruses to 67% (95% CI 54%-76%) against influenza A (H1N1)pdm09 viruses, reported Brendan Flannery, PhD, lead investigator for the CDC’s U.S. Flu Vaccine Effectiveness Network, and colleagues.

Overall vaccine efficacy against influenza A and B associated with medically attended acute respiratory illness was 36% (95% CI 27%-44%), the team wrote in the Morbidity and Mortality Weekly Report.

However, the 25% efficacy against H3N2 strains is higher than earlier estimates suggesting effectiveness as low as 10%.

Asked for his perspective, Peter Hotez, MD, PhD, dean of the National School of Tropical Medicine at Baylor College of Medicine in Houston, who was not involved with the research, highlighted two important figures: that only 26% of pediatric flu deaths occurred in vaccinated children, and that the risk for “medically attenuated” influenza illness was reduced by more than 59% among vaccinated children.

“These results confirm my earlier concerns that public health messaging this year was too focused on the low efficacy of the vaccine in preventing flu infection, rather than preventing flu deaths,” he told MedPage Today. “This message was sadly encouraged by the antivaxxer community. The public health community … in general [was] not sufficiently visible in the media about the importance of getting vaccinated.”

William Schaffner, MD, of Vanderbilt University School of Medicine in Nashville, added that vaccine effectiveness estimates relate to completely preventing infection, but that is not the whole story: “One of the main reasons we use influenza vaccine is to prevent complications, and that’s never measured except in research studies that come later, but they always show that vaccinated people have lower rates of pneumonia hospitalization and dying,” he told MedPage Today.

Schaffner added that the vaccine, while imperfect, “can prevent a lot of infections and modulate many others, and this is a very serious illness, so I’ll take every ounce of prevention I can get.”

Flannery and colleagues wrote that despite “ongoing challenges” with the H3N2 component since the 2011-2012 season, vaccine efficacy was higher in the U.S. for this strain than it was in Australia (10%) and Canada (17%). The U.S. rate was “similar” to the final vaccine efficacy numbers for the 2016-2017 season for this strain, the researchers said.

They emphasized that there is no “definitive evidence” of antigenic drift of viruses circulating this season compared with those included in the vaccine. In addition, egg-based vaccines may be partially responsible for a less effective vaccine against H3N2, but additional studies are needed to assess whether vaccine efficacy varies by vaccine type.

The researchers derived this data from 4,562 children and adults with acute respiratory infection enrolled in five study sites. There were 38% that tested positive for influenza, including 81% that tested positive for influenza A. Among the 1,340 influenza A viruses, 85% were H3N2, and 98% of B viruses were B/Yamagata. The portion of patients vaccinated ranged from 45% to 59% among study sites. Among these participants, 43% of those testing positive for influenza received the 2017-2018 seasonal influenza vaccine versus 53% of influenza-negative participants, the authors said.

There was statistically significant protection against “medically attenuated” influenza among children ages 6 months to 8 years, and among adults 18-49 (VE=33%, 95% CI 16%-47%), but no statistically significant protection among other age groups.

‘Wake Up Call’ for Better Vaccine Development

Stephen Morse, PhD, director of the Infectious Disease Epidemiology Certificate Program of Columbia University Mailman School of Public Health in New York City, told MedPage Today that the only surprising thing about this data was how unsurprising it was. He said he hoped it was “a ‘tipping point’ that will finally galvanize some long-needed improvements in how we produce this vaccine.”

Specifically, Morse noted the strong case for applying these newer and more rapidly adaptable technologies to influenza vaccines, which need to be changed every few years.

“Some vaccines have been produced by modern biotechnology, making just the proteins we need for the vaccine,” he said. “We should bring influenza vaccine production up to date. We can hope that the bad news about lower vaccine effectiveness this year will add momentum to these efforts.”

Schaffner cautioned that although a number of manufacturers are creating their own version of a better vaccine, “waiting for perfection is the enemy of the current good.

“We have more research ongoing to make a better influenza vaccine than we’ve had in the previous 40 years put together. My optimism is that based on reasonable expectations, in 5 to 8 years, we’re going to have more and different influenza vaccines.”

Hotez agreed that investments should be expanded in technologies for both H3N2 and universal flu vaccines, “but even this current vaccine was still effective at preventing deaths from H3N2.”

Flu Was Early and Widespread

A related second report in MMWR, by Alicia P. Budd, MPH, of the CDC, and colleagues, noted that influenza activity began to increase in early November and “rose sharply” from December through February, with “some of the highest levels of influenza-like illness and hospitalization rates in recent years.”

Adults age 65 and older accounted for almost 60% of documented influenza-associated hospitalizations. Among all hospitalizations, over 86% were associated with influenza A, and among patients where subtype was available, over 86% of those were associated with influenza A (H3N2).

Among hospitalized adults for whom information on underlying medical conditions was available, over two-thirds had “at least one medical condition placing them at high risk for influenza-related complications,” with the most common being cardiovascular disease (35.5%). Among hospitalized children with such information, about half had at least one underlying medical condition, with the most common being asthma (22.8%).

The mean age of children who died of influenza-associated causes was 7.4. Among those children with a known medical history, 54% had at least one underlying medical condition that placed them at increased risk of influenza complications. There were 54 children eligible for influenza vaccination, but only 14 had received at least one dose of the vaccine before illness onset.

Morse said that every new development in influenza “makes us painfully aware of how little we really know about this familiar and mundane virus. Better understanding can only help to develop more effective vaccines. The long-standing debates about the effect of repeated immunizations, and how to improve the immune response, show that there is still much we don’t understand about the immunology of influenza infection and how our immune systems respond to the virus.”

Flannery and colleagues disclosed no conflicts of interest.

Budd and colleagues disclosed no conflicts of interest.


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