Is the U.S. Having a Flu Epidemic?

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CDC information on a flu epidemic in the US

Are we in the middle of a flu epidemic?  The United States experiences seasonal flu epidemics each year, and right now all of CDC’s influenza surveillance systems are showing elevated activity. Influenza-like-illness (ILI) has been over baseline for the past several weeks, virological surveillance shows a lot of flu is circulating, and the hospitalization surveillance system shows increasing hospitalizations rates, especially in people 65 years and older. Also, the surveillance system that tracks mortality shows that the country is in the midst of this season’s flu epidemic. During influenza seasons, ILI increases first, and then hospitalizations increase, and then increases in deaths occur, so what is being observed is a typical pattern for the flu season.

The CDC continues to recommend that unvaccinated people get vaccinated. While some of the viruses spreading this season are different from those in the vaccine, vaccination can still provide protection and might reduce severe outcomes such as hospitalization and death. CDC also is reminding clinicians and the public that people with high risk factors who get flu symptoms should be evaluated for possible treatment with flu antiviral drugs. It’s very important that antiviral drugs be used early to treat hospitalized patients, people with severe flu illness, and people who are at higher risk for flu complications based on their age or underlying medical conditions.

Flu Epidemic or Regular Flu Season

Vaccine Effectiveness – How Well Does the Flu Vaccine Work?

Sick_600x400During years when the flu vaccine is not well matched to circulating viruses, it’s possible that no benefit from flu vaccination may be observed. During years when there is a good match between the flu vaccine and circulating viruses, it’s possible to measure substantial benefits from vaccination in terms of preventing flu illness.

When the vaccine viruses for 2014-2015 were selected, A/Texas/50/2012 was the most common circulating influenza H3N2 virus, so it was chosen to be included in the vaccine. The drifted H3N2 viruses that are circulating this season were first detected during routine surveillance testing during late March 2014, after World Health Organization (WHO) recommendations for the vaccine composition for the Northern Hemisphere for the 2014-2015 season had been made (in mid-February). At that time, just a very small number of these viruses had been found among the thousands of specimens that had been collected and tested and there was no way to predict that they would circulate widely.

What is CDC doing to monitor vaccine effectiveness for the 2014-2015 season?

During the 2014-2015 season, CDC is planning multiple studies on the effectiveness of both the flu shot and the nasal-spray flu vaccine. These studies will measure vaccine effectiveness in preventing laboratory-confirmed influenza among persons 6 months of age and older. CDC anticipates early vaccine effectiveness estimates for the 2014-2015 season will be available in mid- to late January.

Flu Vaccine Effectiveness: Questions and Answers for Health Professionals

How does the CDC measure how well influenza vaccines work?

Vaccine efficacy is measured by comparing the frequency of influenza illness in vaccinated and unvaccinated groups of random volunteers.

Vaccine effectiveness is measured by comparing the frequency of influenza illness in vaccinated and unvaccinated volunteer groups, usually with adjustment for factors (like presence of chronic medical conditions) that may vary between the groups.

More information:


by Dan Charobee

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