The 2013-2014 U.S. Flu Season

May 29, 2014

Spring is finally in full swing! Aside from the nicer weather, it also marks the end of flu season. In the U.S., this typically lasts from October to May and peaks in February-but of course every season varies.

The 2013-2014 flu season started in November and peaked in late December, although activity still continues in some areas. Among all the influenza virus strains in circulation, the 2009 H1N1 pandemic virus was, for the first time since the year for which it was named, the most prevalent. During the 2009 pandemic, only an estimated 35% of the population became infected, potentially leaving many unvaccinated individuals at risk for subsequent incursions of the virus, which can persist for many years.

The dominance of the 2009 H1N1 virus this year meant that there were higher-than-expected proportions of illnesses,hospitalizations, and deaths in young- to middle-aged adults as compared to other flu seasons.

The reason comes down largely to patterns of immunity. Adults over age 65 are more likely to have pre-existing immunity to the 2009 H1N1 virus-as was observed during the 2009 pandemic-because of their exposure to strains of similar H1N1 viruses that were circulating before 1957. Secondly, even though the 2009 pandemic strain is included in the seasonal flu vaccine, young- to middle-aged adults have a lower-than-average vaccination rate. The results speak for themselves. According to the U.S. Centers for Disease Control and Prevention, young- to middle-aged adults accounted for about 58% of influenza-related hospitalizations this year, whereas in the three previous seasons, they accounted for, on average, 39%.

Speaking of vaccines, this season vaccine companies produced an estimated 138-145 million doses, which was on par with demand; only about 45% of the U.S. population received a vaccine for 2013-14. Preliminary results showed an efficacy (ability to protect against the flu) around 60%, ranging from 52% for elderly to 67% for children.

An exciting feature of the 2013-2014 flu season (if one can say anything exciting about a flu season!) was the general availability, for the first time, of a vaccine created using "recombinant" technology. Typically, influenza vaccines are created using 50-year old technology that involves growing the flu virus in chicken eggs. This lengthy process leads to a production delay of 6-8 months, and requires that the dominant strains be predicted several months in advance. Recombinant technology uses DNA replication to generate vaccines, and can happen in about two months.

This could have significant implications during a flu pandemic, when a new strain emerges and a vaccine may have to be produced from scratch. Cutting vaccine production time could substantially reduce morbidity and mortality by making vaccines available sooner. However, production capacity for this new technology is currently limited. Only 150,000 doses based on recombinant technology were available for the 2013-2014 flu season. Around 2 - 5 million doses are expected for next season.

Although flu season is starting to wind down in the U.S., things are just now starting to pick up in the Southern Hemisphere, where flu season typically starts in May and lasts until October. And of course there's always the MERS (Middle East Respiratory Syndrome) virus to keep an eye on.

Learn more about influenza pandemic:
Modeling a Modern-Day Spanish Flu Pandemic

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