Flu Season - From Start to Peak and End

Flu Season Basics

A map showing widespread flu during most of the US during the 2009-10 flu season.
In 2009, there was an early flu season, with widespread flu in most of the country by November. Photo courtesy of the CDC

Although flu season is usually thought of as occurring in the winter, the severity and timing of flu season vary from year-to-year. By understanding when you and your family are at risk of contracting influenza, you'll see why it's recommended you get the annual influenza vaccine by the end of October.

Flu Season

Flu season is the time of year when you are most likely to get sick from the flu. In general, flu season can start anytime in late fall, peak in mid-to-late winter (usually January or February), and continue through early spring.

On average, flu season lasts about 13 weeks. It will usually end by April, but in some years it can linger into May. There have been six years in the past where the peak was in March, for example.

It is a good idea to get a flu shot before the start of flu season so that you don't get sick with the flu, But even a late flu shot provides protection, especially when flu season lingers into April or May.

Flu Season Facts

  • Flu season most commonly peaks in February (50 percent of the time).
  • In the Southern Hemisphere (Australia, New Zealand, South Africa, etc.), flu season occurs in the summer months (which is their winter).
  • You can view past flu season reports at the CDC flu website.

Theories About Flu Season

Why is the influenza virus so active in the winter? People used to simply blame the fact that kids are in school and are inside more because of the cold weather, but those were never really satisfying explanations.

Recent research gives an even better explanation. The flu virus is transmitted best at colder temperatures in low humidity, just like we see in the winter.

Details of the Timing and Severity of Past Flu Seasons

See how the flu season has varied. Even in mild years, dozens of children die from influenza, showing the importance of preventing its spread.

The strain of influenza that is circulating can change from year to year, and the vaccine is adjusted yearly in an attempt to predict which will predominate.

  • 2015-16 Flu Season: Late start with 77 pediatric flu deaths.
  • 2014-15 Flu Season: 148 pediatric flu deaths (drifted H3N2 vaccine strain predominate).
  • 2013-14 Flu Season: Began increasing in mid-November and peaked in late December, with 111 deaths in children.
  • 2012-13 Flu Season: An early and intense flu season that lasted slightly longer than usual, with 171 deaths in what the CDC described as a moderately severe flu season. (H3N2 vaccine strain predominate).
  • 2011-12 Flu Season: Was a mild flu season, but still saw 37 deaths in children.
  • 2010-11 Flu Season: Started in December, peaked in February and ended in April, with 123 deaths in children.
  • 2009-10 Flu Season: 2009 H1N1 pandemic flu virus continued from the summer and fall of 2009, peaked in October, and ended in December, with 282 deaths in children.
  • 2008-09 Flu Season: Seasonal flu season started in December, peaked in February, and ended in April. But then cases of the 2009 H1N1 virus began in May 2009 and continued throughout the summer and fall of 2009, with 133 deaths in children.
  • 2007-08 Flu Season: Started in January, peaked in February and ended in April. It was considered to be more severe than the previous three flu seasons, with 88 deaths in children (drifted H3N2 vaccine strain predominate).
  • 2006-07 Flu Season: Started in December, peaked in February and ended in April, with 68 deaths in children.
  • 2005-06 Flu Season: Started in February, peaked in March, and ended in April, but was considered a mild flu season, with 41 deaths in children.
  • 2004-05 Flu Season: Started in December, peaked in February and ended in April, with 39 deaths in children.
  • 2003-04 Flu Season: Started in October, peaked in November and ended in December. It was considered to be a moderately severe flu season, with 152 deaths in children (H3N2 vaccine strain predominate).

    Bottom Line on Flu Season

    The CDC recommends that everyone over the age of 6 months get the yearly flu vaccine and that you get it as soon as it becomes available, by the end of October, if possible. It takes about two weeks after vaccination for your body to produce the antibodies that will protect you from the flu. But if you missed the earlier vaccination time, it is still valuable to get the vaccine even in January or later.

    Source:

    Past Weekly Surveillance Reports. CDC. https://www.cdc.gov/flu/weekly/pastreports.htm

    The Flu Season. CDC. https://www.cdc.gov/flu/about/season/flu-season.htm.

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