2009 H1N1 Flu In The News

February 12, 2010 1:00 PM ET

How many 2009 H1N1 cases, hospitalizations and deaths are estimated to have occurred in the United States?

On February 12, 2010, CDC issued updated estimates of the number of 2009 H1N1 cases, hospitalizations and deaths for the United States from April 2009 through January 16, 2010.

• CDC estimates that between 41 million and 84 million cases of 2009 H1N1 occurred between April 2009 and January 16, 2010. The mid-level in this range is about 57 million people infected with 2009 H1N1.


• CDC estimates that between about 183,000 and 378,000 H1N1-related hospitalizations occurred between April 2009 and January 16, 2010. The mid-level in this range is about 257,000 2009 H1N1-related hospitalizations.


• CDC estimates that between about 8,330 and 17,160 2009 H1N1-related deaths occurred between April 2009 and January 16, 2010. The mid-level in this range is about 11,690 2009 H1N1-related deaths.

CDC first provided estimates for the number of 2009 H1N1 cases, hospitalizations and deaths for April through October 17, 2009 on November 12, 2009 and committed to updating those estimates approximately monthly. Updated estimates for April through November 14, 2009 were issued on December 10, 2009 and updated estimates for April through December 12, 2009 were issued on January 15, 2010. The same methodology was applied to derive each set of estimates.

The latest estimates incorporate an additional 5 weeks of flu data (from December 13, 2009 through January 16, 2010) that correlates with a five week period of generally low flu activity in the United States. The latest estimates show a relatively small increase in the total number of 2009 H1N1 cases, hospitalizations and deaths since the previous estimates posted on January 15, 2010.

A table showing this data by age group is available. In addition, background information on these estimates and information about the methodology used to generate these estimates also is available.

Which age groups have been most impacted by 2009 H1N1, according to CDC’s estimates?

CDC’s latest estimates of 2009 H1N1-associated cases, hospitalizations and deaths continue to highlight the fact that people younger than 65 years of age have been more affected by 2009 relative to people 65 and older. This is in stark contrast to seasonal flu, where about 60 percent of seasonal flu-related hospitalizations and 90 percent of flu-related deaths occur in people 65 years and older. With 2009 H1N1, about 90% of estimated hospitalizations and 87% of estimated deaths from April 2009 through January 16, 2010 occurred in people younger than 65 years old. While there have been severe infections and deaths from 2009 H1N1 in every age group, including people 65 and older, the largest burden has fallen on people younger than 65. In addition, data from CDC’s Emerging Infections Program on flu-related hospitalizations this season indicate that about 80% of adults and about 60% of children who have been hospitalized with 2009 H1N1 have had underlying health conditions.

What does this data indicate with regards to 2009 H1N1 vaccination recommendations?

This data supports the recommendations made in July by the the Advisory Committee on Immunization Practices (ACIP) that initial doses of 2009 H1N1 vaccine be prioritized for people who were more likely to get infected and become very ill. This included all children and young adults 6 months through 24 years old, pregnant women, and adults 25 through 64 years of age with a health condition associated with higher risk of medical complications from flu. People 65 and older were not prioritized for vaccination initially because they were less likely to become sick relative to people in other age groups. It was recognized, however, that people age 65 and older were at high risk for serious complications if they were to become ill and for that reason, people in that age group were prioritized to get antiviral drugs if they became sick. The rationale for how the initial target groups for vaccination were identified is described in the transcript  (1MB) and slide presentations from the July ACIP meeting. The rationale behind this decision was again covered in an ACIP meeting in October 2009 and the slides from this meeting are publicly available as well. The full ACIP recommendations on the use of 2009 H1N1 vaccine are available online. While people 65 and older were not included in the initial groups recommended to get the earliest doses of vaccine, as soon as supplies of the vaccines to protect against the 2009 H1N1 virus were sufficiently large, CDC encouraged programs to opened up vaccination to anyone who wanted it, including people 65 and older. A special day (Friday, January 15, 2010) during National Influenza Vaccination Week (NIVW) was designated to raise awareness about the importance of vaccination in people 65 years and older. CDC continues to recommend that anyone 6 months or older who wants to be protected from the 2009 H1N1 virus be vaccinated, regardless of age.