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Influenza vaccine 48% effective in US
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New interim estimates indicate that the influenza vaccine is 48% effective in the United States. Meanwhile, influenza activity remains elevated in many parts of the country, and the number of deaths attributed to pneumonia and influenza continue to be higher than the epidemic threshold, according to a pair of CDC reports.
“Annual influenza vaccination is the first and best defense to protect against influenza infection,” Lenee Blanton, MPH, of the CDC’s Influenza Division, National Center for Immunization and Respiratory Diseases, and colleagues wrote in a recent MMWR. “Even during seasons when [vaccine effectiveness (VE)] is reduced, vaccination can offer substantial benefit and might reduce the likelihood of severe outcomes such as hospitalization and death.”
A (H3N2) influenza vaccines less effective than vaccines for other strains
Brendan Flannery, PhD, epidemiologist in the CDC’s Influenza Division, and colleagues, analyzed data on 3,144 children and adults with acute respiratory illness (ARI) enrolled in the Influenza Vaccine Effectiveness Network from Nov. 28, 2016, to Feb. 4, 2017, to determine vaccine efficacy. Of the 24% of participants who tested positive for influenza, 88% were infected with influenza A. The majority of influenza A viruses (98%) were A (H3N2).
An analysis adjusted for study site, age, gender, race/ethnicity, self-rated health and number of days from illness onset to enrollment revealed that VE against ARI due to influenza was 48% (95% CI, 37%-57%). Specifically, the vaccine was 43% (95% CI, 29%-54%) effective against A (H3N2) and 73% effective against influenza B virus infection. The only age groups with significant protection against A (H3N2) were children aged 6 months to 8 years (VE = 53%; 95% CI, 16%-74%) and adults aged 50 to 64 years (VE = 50%; 95% CI, 23%-67%).
“Since the 2009 influenza A (H1N1) pandemic, VE estimates for A (H3N2) viruses have been lower than VE estimates against A (H1N1) and influenza B viruses,” Flannery and colleagues wrote.
According to the researchers, the interim VE estimates for the current season are similar to those during the 2011 to 2012 and 2012 to 2013 influenza seasons, which were estimated to be 39%. A meta-analysis of studies evaluating VE from the 2007 to 2008 and 2014 to 2015 seasons showed that the vaccine was approximately 33% effective against A (H3N2) vs. 61% effective against A (H1N1) and 54% effective against influenza B.
“These results reflect properties unique to A (H3N2) viruses that pose special challenges,” the researchers wrote. “Influenza A (H3N2) viruses undergo more frequent and extensive genetic changes than do influenza A (H1N1) and influenza B viruses, and require more frequent updates to the A (H3N2) vaccine virus components to maintain activity against evolving circulating strains. In addition, A (H3N2) viruses continue to undergo changes in their receptor-biding specificity, which might result in genetic changes during growth in eggs. Efforts are ongoing to improve influenza vaccine effectiveness against A (H3N2) viruses.”
Influenza activity remains elevated in US
In a separate report, Blanton and colleagues examined data from Oct. 2, 2016, to Feb. 4, 2017, on influenza activity in the United States, which increased in mid-December and remained elevated throughout the observation period. According to the data, 38,244 of the 392,901 respiratory specimens tested in clinical laboratories and 15,781 of 38,141 respiratory specimens tested in public health laboratories were positive for influenza. More than 92% of viruses in public health laboratories were influenza A.
Additional information was available for 14,335 influenza A specimens and 851 influenza B specimens from public health laboratories. Most influenza A viruses were H3N2, representing 97.5% of viruses. Meanwhile, 2.5% of strains were influenza A (H1N1)pdm09. Among the influenza B specimens, 54.1% were of the B/Yamagata lineage and 45.9% were of the B/Victoria lineage.
Researchers tested 807 influenza virus specimens for resistance to antiviral medications, including Tamiflu (oseltamivir, Roche), Relenza (zanamivir, GlaxoSmithKline) and Rapivab (peramivir, BioCryst Pharmaceuticals). All viruses were sensitive to all three antivirals.
A surveillance network of approximately 2,000 centers reported that the weekly percentage of outpatient visits for influenza-like illnesses (ILI) ranged from 1.2% to 4.8%, exceeding the national baseline percentage for 8 consecutive weeks. High influenza activity was reported in 23 states, including Alabama, Arkansas, Connecticut, Georgia, Hawaii, Indiana, Kansas, Louisiana, Minnesota, Mississippi, Missouri, New Jersey, New Mexico, New York, North Carolina, Oklahoma, Pennsylvania, South Carolina, South Dakota, Tennessee, Texas, Virginia and Wyoming.
The CDC identified 6,804 laboratory-confirmed hospitalizations linked to influenza. Patients aged 65 years and older accounted for the highest rate of hospitalizations at 60%. The overall hospitalization rate for all age groups was 24.3 per 100,000 population.
Information from the National Center for Health Statistics (NCHS) Mortality Reporting System showed that 7.9% (2,691 of 33,868) of all U.S. deaths that occurred during the week ending on Jan. 21 were attributed to pneumonia and influenza, which is higher than the epidemic threshold for this period (7.4%). This is the third consecutive week where the national average exceeded the epidemic threshold. So far this season, 20 laboratory-confirmed deaths among children were reported.
Two novel influenza A viruses identified
The CDC reported that, for the first time, a person was infected with an avian lineage influenza A (H7N2) virus that was likely acquired through exposure to an infected cat. The case was identified during an investigation of an A (H7N2) outbreak among cats in animal shelter in New York City in December. Approximately 350 people exposed to the cats were tested, but only one human infection was reported. It is the first A (N7N2) virus detected in a human in the United States since 2003.
“The finding of an avian lineage influenza virus in an unexpected host, such as a domestic cat, or any human infection with a nonhuman influenza virus is concerning,” Blanton and colleagues wrote.
According to a statement issued by the CDC on Dec. 22, the risk for transmission of avian influenza infections among humans is low but likely increases with exposure duration and intensity.
“It is possible that additional human infections could be detected in association with this outbreak in cats,” the statement said. “The risk posed by this virus to the public (people who have no contact with infected cats or an infected person) is thought to be very low at this time.”
Meanwhile, another patient from Iowa who reported exposure to swine the week before illness was infected with a novel influenza A (H1N2) variant [(H1N2)v] virus. Neither of these patients were hospitalized, and there was no evidence of ongoing human-to-human transmission of the viruses.
“Early identification and investigation of human infections with novel influenza A viruses are critical so that the risk of infection can be more fully understood and appropriate public health measure can be taken,” Blanton and colleagues wrote. “Public health laboratories should immediately send virus specimens that they cannot subtype using standard methods to CDC and submit all specimens that are otherwise unusual as soon as possible after identification.” – by Stephanie Viguers
References:
Blanton L, et al. MMWR Morb Mortal Wkly Rep. 2017;doi:10.15585/mmwr.mm6606a2.
Flannery B, et al. MMWR Morb Mortal Wkly Rep. 2017; doi: 10.15585/mmwr.mm6606a3.
Disclosure: The researchers report no relevant financial disclosures.
Perspective
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Donald Kaye, MD, MACP
Avian influenza A (H7N2) has caused outbreaks in poultry in the United States and elsewhere in the world. Most of the outbreak strains have been of low pathogenicity for the birds. Human cases have occurred producing mild illness. There was an outbreak in the United Kingdom in 2007 in which 17 people exposed to poultry developed infection with H7N2. Conjunctivitis was the usual symptom. Cats are susceptible to influenza, including avian influenza strains. This outbreak, affecting a large number of cats in a shelter, may have started with a cat that attacked an infected bird and bit off more than it could chew. The human casualty was a mildly ill veterinarian. The concern is always the possibility of mutation or recombination of the avian strain with a human strain in a coinfected person (or animal) to produce a more dangerous influenza virus.
References:
ProMED-mail. 2004. Avian influenza A(H7N2) virus, human – USA (New York). Archive no. 20040420.1104. http://www.promedmail.org.
ProMED-mail. 2007. Avian influenza H7N2, human – United Kingdom (WALES). Archive no. 20070606.1830. http://www.promedmail.org.
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