H1N1 Flu

Content on this page was developed during the 2009-2010 H1N1 pandemic and has not been updated.

The H1N1 virus that caused that pandemic is now a regular human flu virus and continues to circulate seasonally worldwide.
The English language content on this website is being archived for historic and reference purposes only.
For current, updated information on seasonal flu, including information about H1N1, see the CDC Seasonal Flu website.

2009 H1N1 Flu: International Situation Update

March 19, 2010, 4:00 PM ET

Map of International Activity Estimates
(Including 2009 H1N1 Flu)

This report provides an update to the international flu situation using data collected through March 14, 2010, and reported by the World Health Organization (WHO) on March 19. WHO continues to report laboratory-confirmed 2009 H1N1 flu cases and deaths on its Web page. These laboratory-confirmed cases represent a substantial underestimation of total cases in the world, as most countries focus surveillance and laboratory testing only on people with severe illness.

In nearly all countries of the world where influenza virus infections have been reported, the 2009 H1N1 virus continues to predominate among all subtyped influenza A viruses. Based on FluNet data collected by 28 countries from February 28 to March 6, 2010, 90.2% of all subtyped influenza A viruses were 2009 H1N1 positive. Among specimens that tested positive for influenza, 34.0% were typed as influenza A and 66.0% as influenza B.

Transmission of 2009 H1N1 virus continues to be the most active in Southeast Asia and West Africa. Limited data suggests that influenza activity may also be increasing in certain areas of the Caribbean and Central America. Low levels of 2009 H1N1 influenza activity continue to circulate across South and Southeast Europe and East, West, and South Asia. Although 2009 H1N1 virus continues to be the predominant influenza virus circulating worldwide, seasonal influenza B viruses are predominate in East Asia, and have been detected at low levels across Southeast Asia and Eastern Africa. 2009 H1N1 activity remains low in the temperate zone of the Southern Hemisphere.

Selected Highlights

According to WHO, the majority of 2009 H1N1 virus isolates tested worldwide remain sensitive to oseltamivir, an antiviral medicine used to treat flu. Among 2009 H1N1 isolates tested worldwide, 267 have been found to be resistant to oseltamivir – 61 of these isolates were detected in the United States.
In recent weeks, influenza B has been the predominate subtype in China, Hong Kong (China SAR), Iran, Mongolia, the Republic of Korea and the Russian Federation. Increased influenza B activity has also been observed in some European countries.
On February 18, 2010, WHO published recommendations for the following viruses to be used for influenza vaccines in the 2010-2011 influenza season of the Northern Hemisphere:

an A/California/7/2009 (H1N1)-like virus;
an A/Perth/16/2009 (H3N2)-like virus*;
a B/Brisbane/60/2008-like virus.

* A/Wisconsin/15/2009 is an A/Perth/16/2009 (H3N2)-like virus and is a
2010 Southern Hemisphere vaccine virus.

International Resources for 2009 H1N1 Information

Health Organizations

World Health Organization (WHO) Regional Offices

Travel and 2009 H1N1 Flu

Human cases of 2009 H1N1 flu virus infection have been identified in the United States and several countries around the world. For information on 2009 H1N1 flu and travel, see the CDC H1N1 Flu and Travel website.

Reports and Publications

White House Report on 2009 H1N1 in the Southern Hemisphere
Issued August 2009 – This White House report was prepared by the Department of Health and Human Services (HHS) in coordination with the Office of the Director for National Intelligence (ODNI) and the Department of State (Dos) and describes the characteristics and impact of 2009 H1N1 influenza A virus in the Southern Hemisphere.
ECDC Interim Risk Assessment Influenza A (H1N1) 2009 Pandemic
Issued July 30, 2009 - This document provides an interim risk assessment of novel H1N1 flu in Europe prepared by ECDC.
World Health Organization Weekly Epidemiological record – Issued July 24, 2009
This document by WHO provides updates on the international novel H1N1 flu situation.
MMWR – Update: Novel Influenza A (H1N1) Virus Infection – Mexico, March-May, 2009 – Issued June 5, 2009 / Vol. 58 / No. 21.
This Morbidity and Mortality Weekly Report describes the novel influenza A (H1N1) outbreak in Mexico from March-May, 2009.
MMWR – Update: Novel Influenza A (H1N1) Virus Infections – Worldwide, May 6, 2009 – Issued May 8, 2009 / Vol. 58 / No. 17.
This Morbidity and Mortality Weekly Report describes worldwide novel influenza A (H1N1) infections as of May 6, 2009.