The World Health Organization (WHO) reported today that 225 cases of H1N1 flu with resistance to oseltamivir (Tamiflu) have been found worldwide, and resistant viruses have spread from person to person in several clusters but have not spilled into the community.

Many of the resistant cases involved people with severely weakened immunity, reinforcing the importance of monitoring for the problem in such patients, the WHO said in today´s issue of its Weekly Epidemiological Record.

The 225 cases come from 20 countries and include 65 cases in the Americas, 77 in Europe, 1 in Africa, and 82 in the Western Pacific region, the agency said. All the isolates had the H275Y mutation that confers resistance to oseltamivir but not to the other neuraminidase inhibitor in general use, zanamivir (Relenza).

Of 142 cases for which data were available, 56 (40%) were in severely immunocompromised patients, and 54 (38%) were linked to treatment of flu. Another 16 cases (11%) were associated with preventive treatment, and 16 others did not involve any known antiviral use.

The WHO reviewed reports of three clusters of resistant cases, one each in the United States, the United Kingdom, and Vietnam. They involved:

• Four severely immunocompromised patients in the same ward at Duke University Hospital in Durham, N.C., in October and November; in three cases, resistance was found before the drug was used
• Eight patients with hematological malignancies at the University Hospital of Wales in Cardiff, Wales, in November, at least four of whom contracted the resistant virus from someone else
• Seven healthy young adults in Vietnam who traveled together by train last July and were believed to have caught the virus from an unidentified index case; the cluster apparently did not lead to any further cases

The WHO said the number of resistant cases remains low despite the large scale of the pandemic, the wide use of oseltamivir, and extensive monitoring.

"Although there is no evidence of general community circulation of such resistant viruses, there is clear evidence of limited person-to-person transmission in several epidemiological settings," the agency said. It added that active surveillance for resistant cases should continue and all cases should be investigated and reported to health authorities.

See also:

WHO. Update on oseltamivir-resistant pandemic A (H1N1) 2009 influenza virus: January 2010. Wkly Epidemiol Rec 2010 Feb 5;85(6):37-40 [Full issue]

Nov 20, 2009, CIDRAP News story "Clusters of resistant H1N1 cases reported in UK, US"