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

April 23, 2010, 5:30 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 April 18, 2010, and reported by the World Health Organization (WHO) on April 23. 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.

Currently, the most active areas of 2009 H1N1 transmission are in parts of West and Central Africa. Influenza activity is still widely circulating in Southeast Asia and Central America.Overall, influenza activity remains low in the Northern Hemisphere. In the temperate zone of the Southern Hemisphere, Chile reported increases in ILI activity for the past four weeks. In Australia and New Zealand, there is no evidence yet of the start of transmission of influenza viruses. Influenza type B viruses are now the predominant circulating influenza viruses across East Asia, Central Africa and Northern and Eastern Europe. Very small numbers of type B viruses have also been detected in Central America. Seasonal influenza H3N2 activity is still being reported in South and Southeast Asia, as well as in several countries of West Africa and Eastern Europe.

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, 285 have been found to be resistant to oseltamivir – 64 of these isolates were detected in the United States. Approximately 1.2% of U.S. 2009 H1N1 viruses tested by CDC since September 30, 2009, have been resistant to oseltamivir.
Influenza B was reported as the predominat influenza virus accounting for 82.1% of all influenza detections in the Russian Federation, 89.5% in China, 92.9 % in Sweden, 97.8% in the Republic of Korea, and 100% in Iran, Latvia, and Mongolia.Intermittent influenza H3N2 activity was also reported from certain countries including China, Japan, and the Russian Federation.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.