2014-15 Flu Vaccine Efficacy Low, But Still Recommended

At the end of February, the Centers for Disease Control and Prevention (CDC) reported that the 2014-15 seasonal influenza vaccine was just 18 percent effective against the dominant strain of flu — a drop from the 23 percent protection level estimated earlier in the season. What’s worse is that CDC estimated the effectiveness of the injected vaccine for kids age 2 years to 8 years at just 15 percent, and the nasal spray version of the vaccine not protective in young children at all. Why is this year’s flu vaccine protection rate so low? Identifying the vaccine strains’ potential protective benefit is one of the most troublesome issues surrounding influenza vaccines. However, even during seasons when there is a less than ideal match, CDC still recommends the flu vaccine for everyone 6 months and older, and it emphasizes the particular importance of the vaccine for people at high risk for serious flu complications and their close contacts.

The 2014-15 Mismatch

Twice annually, the World Health Organization (WHO) consults with an advisory group of experts to analyze influenza virus surveillance data generated by the WHO Global Influenza Surveillance and Response System, and then issues recommendations on the composition of the influenza vaccines for the following influenza season. These recommendations are used by the national vaccine regulatory agencies and the pharmaceutical companies to develop, produce and license influenza vaccines months in advance so that manufactures have time to make the vaccines. But, predicting which strains of the virus to include in the influenza vaccines has been difficult both because the virus antigenically drifts (mutates) from year to year and the number of influenza subtypes A and type B that can be selected for inclusion is limited.

Until the 2013-14 season, the influenza vaccine was a trivalent vaccine (IIV3) and contained only three strains: two A strains and one B strain. But, since the year 2000, two influenza B lineages (Victoria and Yamagata) have co-circulated, and various degrees of mismatch have occurred between the B lineage included in IIV3s and the B lineage that actually circulated, causing an increased risk of influenza-related morbidity across all age groups. Therefore, it was thought that with FDA approval of the new quadrivalent vaccine (IIV4) that added a fourth strain (a second type B strain), the chances of predicting the correct strains should improve.

For 2014–15, the IIV3 influenza vaccines contained the same vaccine virus strains as those in the 2013–14 vaccine: an A/California/7/2009 (H1N1)-like virus, an A/Texas/50/2012 (H3N2)-like virus, and a B/Massachusetts/2/2012-like (Yamagata lineage) virus. The IIV4 influenza vaccines contained those strains, as well as a B/Brisbane/60/2008-like (Victoria lineage) virus. But, this season, the mismatch occurred among one of the A strains, with influenza A (H3N2) viruses reported most frequently in almost all states. The H3N2 virus that is currently circulating drifted to look very different from the vaccine strains chosen. According to CDC, “During past seasons when influenza A (H3N2) viruses have predominated, higher overall and age-specific hospitalization rates and more mortality have been observed, especially among older people, very young children, and persons with certain chronic medical conditions compared with seasons during which influenza A (H1N1) or influenza B viruses have predominated.” Unfortunately, this drifted strain was not identified until March and didn’t become dominant until September, which was far too late to make new vaccines. “This is a very unusual circumstance where a new strain develops and becomes a dominant strain so quickly and after the vaccine has already been produced,” said Dr. Joseph Bresee, chief of Epidemiology and Prevention Branch at CDC’s National Center for Immunization and Respiratory Diseases.

On Feb. 26, WHO recommended that this year’s drifted H3N2 strain, as well as updated versions of other strains be included in next season’s vaccine. Trivalent vaccines for use in the 2015- 2016 influenza season (northern hemisphere winter) will contain an A/California/7/2009 (H1N1)pdm09-like virus, an A/Switzerland/9715293/2013 (H3N2)-like virus, and a B/Phuket/3073/2013-like virus. Quadrivalent vaccines will contain those three viruses and a B/Brisbane/60/2008-like virus. On March 4, the Vaccines and Related Biological Products Advisory Committee voted to adopt WHO’s recommendations.

2014-15 Vaccine Still Protective

While the 2014-15 flu vaccine is not a good match, CDC still recommends people get vaccinated because “antibodies made in response to one flu virus can sometimes provide protection against different but related viruses. A less than ideal match may result in reduced vaccine effectiveness against the virus that is different from what is in the vaccine, but it can still provide some protection against influenza illness.” In addition, the flu vaccine contains three or four flu viruses (depending on the type of vaccine received), so even when there is a less than ideal match or lower effectiveness against the virus, the vaccine may protect against the other viruses.