Current flu vaccine protects about half of recipients

With influenza season still in full swing, the Centers for Disease Control and Prevention (CDC) said today that interim influenza vaccine effectiveness estimates suggest this year's vaccine protects roughly half of recipients. These numbers are similar to a report released today in Europe.

According to new data published in Morbidity and Mortality Weekly Report (MMWR), the overall estimated vaccine effectiveness (VE) of the 2016-17 seasonal influenza vaccine for preventing laboratory-confirmed influenza virus infection was 48%.

"This is far from ideal, but it's better than no protection at all," said Ed Belongia, MD, director of the Center for Clinical Epidemiology & Population Health at Marshfield Clinic Research Foundation and coauthor of the study. "But this is the peak of flu season, we certainly don't want to discourage anyone from getting vaccinated if they haven't done so yet."

Belongia explained that this year's flu is dominated by H3N2, a strain that has a poor track record with flu vaccines.

"H3N2 seasons tend to be more severe, compared to [last year's] H1N1 season," said Belongia. "Generally vaccine protection for H3N2 is consistently lower, in the range of 30% to 50%."

The CDC's data came from 3,144 children and adults with acute respiratory illness enrolled at five study sites (outpatient clinics) from Nov 28, 2016, to Feb 4, 2017. VE was 43% against the predominant influenza A (H3N2) virus and 73% against illness caused by influenza B virus. Influenza A caused 88% of the cases studied, which mirrors the national trend.

Elderly most sickened by H3N2

The flu season began in earnest in mid-December, and by Feb 4 was at its current levels, according to the CDC's update on influenza activity, also published in this week's MMWR.

The update included information on age and influenza. Of the 13,306 influenza-positive patients monitored, 7.9% were ages 0 to 4 years, 30.4% were between 5 and 24 years, 4, 30.3% were aged 25 to 64 years, and 31.5% were older than 65.  Each age-group was dominated by influenza A infections, ranging 82.3% of cases in patients ages 0 to 4 to 93.6% in the oldest cohort. The largest number of influenza B viruses were reported in persons ages 5 to 24 years. As of Feb 4, there have been 20 pediatric deaths due to influenza in the United States.

Across the United States, the percentage of outpatient visits for influenza-like illness (ILI) was 4.8%, well-over the national baseline (2.2%). The vast majority of flu hospitalizations were due to influenza A virus (93.6%). And those over the age of 50 had the highest hospitalization rates.

Similar trends in Europe with 'suboptimal' vaccine

Reports from European outpatient clinics and hospitals also show a middling vaccine performance this season.

In today's issue of Eurosurveillance, researchers write that VE against  H3N2 was 44.1%, 46.9%, and 23.4% among those in the 0-to-14, 15-to-64, and 65-and-older age-groups. They also calculated a VE of 25.7% in an influenza vaccination target group, such as older people or those with underlying health conditions.  

This trend echoes what was seen in previous H3N2-dominant seasons, such as 2012-13. The authors of the study said the vaccine was performing "suboptimally" and that hospitalizations of elderly flu patients were increasing.

Need for better vaccines

"These numbers are the new normal, and unfortunately I don't believe this is an acceptable normal," said Michael Osterholm, PhD, MPH, director of the Center for Infectious Disease Research and Policy at the University of Minnesota, publisher of CIDRAP News. "When we see these trends year after year, it should scream that we need a massive effort to replace the flu vaccine with something more effective."

While Belongia said that there was a "glass half-full or half-empty" way to look at the numbers, he repeated the CDC's guidelines, which suggest anyone over 6 months still get vaccinated. As of Feb 3, the CDC said that approximately 145 million doses of influenza vaccine had been distributed.

"It's far better than having no protection at all," said Belongia. "And any reduction is having a big public health impact."

Belongia said the flu research community is aware that better vaccines need to be developed, but right now, "nothing is waiting in the wings to replace the current vaccine product."

See also:

Feb 16 MMWR report on interim flu VE estimates

Feb 16 MMWR update on US flu season

Feb 15 Eurosurveill study

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