What to Expect for the Influenza Season

Patricia Yarberry Allen, M.D. is a Gynecologist, Director of the New York Menopause Center, Clinical Assistant Professor of Obstetrics and Gynecology at Weill Cornell Medical College, and Assistant Attending Obstetrician and Gynecologist at New York-Presbyterian Hospital. She is a board certified fellow of the American College of Obstetrics and Gynecology. Dr. Allen is also a member of the Faculty Advisory Board and the Women’s Health Director of The Weill Cornell Community Clinic (WCCC). Dr. Allen was the recipient of the 2014 American Medical Women’s Association Presidential Award.

Influenza 2018 – 2019 Season

Last winter, an estimated 80,000 Americans died of the flu and its complications according to the Centers for Disease Control and Prevention (CDC). It was the highest number of deaths since 1976. Lack of immunization among adults and children contributed to this increase in death from the flu. One year ago, 60% of Americans had not received the 2017-2018 seasonal flu shot. As many as 80% of the children who died as a result of the flu had not received a flu vaccination. In addition, last year’s flu vaccine was less protective than it had been in many previous years. In part, this was because the vaccine was not as effective against one of the predominant circulating strains of influenza. It’s now believed, according to a statement by the Food and Drug Administration (FDA) Commissioner, Scott Gottlieb, M.D., that part of the reason that the vaccine was not as protective was that the flu strain used to manufacture the vaccine mutated very subtly during the development process. The result of these small, but very targeted changes, was that the protective immune “antibodies” that the flu vaccine elicited were slightly different than what people needed to mount the most effective immune response to the virus.

What causes Influenza?

In 1933, researchers discovered that viruses cause influenza in humans and isolated the human influenza virus (Smith, W., C.H. Andrewes, and P.P. Laidlaw. 1933. Lancet. 222:66–68). Viruses are much smaller than bacteria. All they have is a protein coat and a core of genetic material, either RNA or DNA. Unlike bacteria, viruses can’t survive without a host organism. Viruses can only reproduce by attaching themselves to cells. In most cases, they reprogram these infected cells to make new viruses until the cells burst, releasing millions of new viruses, killing the cell where they reproduced.

Seasonal flu is caused by influenza virus types A and B primarily. Influenza A and B viruses are genetically similar enough to be included in the same family of viruses, however, their genetic differences are significant enough so that these are separated into two different types. Influenza A viruses are further categorized by subtype and strain. Influenza B viruses are categorized only by strain. This difference is due to the more rapid mutation rate seen in influenza A viruses compared to influenza B viruses. Influenza A viruses are by far the most important human influenza pathogens even though Influenza B viruses circulate widely only among humans.

What are the main factors that influence which viral strains are selected for use in Influenza vaccine production?

The seasonal flu vaccine protects against the influenza viruses that research indicates will be most common during the upcoming season. To select the influenza virus strains for inclusion in each season’s flu vaccine, the FDA, World Health Organization (WHO), the (CDC) and other partners review data collected on the circulating strains of influenza from around the world to identify those strains that are likely to cause the most illness during the flu season in the U.S. Attention is paid to the flu patterns in the southern hemisphere, which always has its flu season before the U.S. Since it generally takes several months for influenza vaccines to be produced, flu strains for the next season need to be selected months in advance to ensure that enough flu vaccinations have been produced and are available when the flu season begins.

According to a statement released on September 27, 2018, by FDA Commissioner Gottlieb, “We’ve worked closely with our colleagues at the CDC and other agencies to ensure that as many of the potential issues with last year’s vaccine as possible were addressed this year. This includes replacement of influenza A H3N2 strain used in 2017-2018 (A Hong Kong) with a new strain (A Singapore) that will hopefully be a better match for the circulating strains in 2018-2019. One of the 2017-2018 influenza B strains (B/Brisbane) has also been replaced for 2018-2019 with a potentially better match as well (B/Colorado).”

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