New CDC Report Gives Insight Into Effectiveness of This Season's Flu Vaccine

With the approach of the annual flu season in the United States, health surveillance experts are keeping an eye on new influenza data coming in. A recent report from the Centers for Disease Control and Prevention (CDC) shows influenza viruses currently in circulation in the United States are the same as anticipated by vaccine makers, hopefully signaling effectiveness ahead for 2016-2017 Northern Hemisphere influenza vaccines.

The CDC’s Morbidity and Mortality Weekly Report (MMWR) is a weekly series of timely public health information and recommendations read mostly by physicians, nurses, health practitioners, epidemiologists, and other scientists and researchers. Using data gathered from state health departments, the recent report on influenza activity in the United States offers a look into what the country can expect for the upcoming flu season based on current surveillance on the virus.

According to the CDC, over a period of 31 seasons between 1976 and 2007, estimates of flu-associated deaths in the United States ranged from a low of about 3,000 to a high of about 49,000 people, mostly aged 65 years and older. While there are four types of influenza viruses—A, B, C and D—the A and B viruses are the ones that cause seasonal illness. The two most common strains of influenza A are H1N1 and H3N2, denoted by the subtypes hemagglutinin and neuraminidase proteins on the surface of the virus. The influenza B lineages that most commonly causing flu outbreaks are B/Yamagata and B/Victoria.

The new MMWR report from the CDC details findings by public health laboratories on influenza viruses currently circulating. Overall, surveillance findings from May 22 to September 10, 2016, show that influenza activity was low and below epidemic thresholds, with influenza A (H3N2) and influenza A (H1N1) co-circulating with influenza B. After testing 5,365 specimens for influenza, they found 817 that were positive for seasonal influenza viruses—458 (56.1%) were influenza A viruses and 359 (43.9%) were influenza B viruses. Of the 448 influenza A viruses subtyped, 377 (84.2%) were influenza A (H3N2) viruses and 71 (15.8%) were influenza A (H1N1) viruses. Of the 249 influenza B viruses with lineage determined, 172 (69.1%) were B/Yamagata lineage and 77 (30.9%) were B/Victoria lineage.

Each year, vaccine makers study antigenic and genetic characterization of influenza viruses to determine vaccine components for the upcoming flu season. It is impossible to predict which influenza virus will predominate to cause illness outbreaks each year, due to the potential for antigenic drift from season to season. When influenza A (H3N2) viruses collected in late summer of the 2014-2015 season showed significant antigenic shift compared to that year’s recommended vaccine, it signaled that there would be a low level of vaccine effectiveness for the coming flu season.

Twice a year, the World Health Organization sponsors a vaccine consultation meeting, where world health leaders meet to review influenza surveillance data and determine vaccine components. At the February 2016 meeting, attendees selected the components for the 2016-2017 Northern Hemisphere influenza vaccine. Since then, the CDC’s laboratory-based studies of approximately 5,000 influenza viruses found that most circulating viruses do not have significant changes and have been characterized antigenically, genetically, or both as being similar to the reference viruses representing vaccine components recommended. This gives a good indication on this season’s influenza vaccine's ability to produce an immune response against circulating influenza viruses.

The CDC recommends getting a flu shot as the best way to avoid an influenza infection each season. Vaccines need to be given each year as the immunity goes away over time and the components of a previous season’s vaccine may not offer adequate protection from viruses circulating in following seasons. While the effectiveness of the vaccine can vary, flu shots reduce the number of influenza-related illnesses, doctor visits, and hospitalizations.