Data also released on efficacy of masks, new antiviral treatments

SAN FRANCISCO | Research on the influenza A(H1N1) pandemic strain dominated presentations at the 49th Interscience Conference on Antimicrobial Agents and Chemotherapy (ICAAC) here.

With the exception of the last session on the last day of the conference, all H1N1 sessions were standing room only, and overflow rooms had to be found.

Among the key findings:

Infectiousness: According to studies in Quebec City and in Singapore, it appears that a substantial proportion of patients infected with H1N1 may shed virus for longer than has been seen with seasonal flu strains—even if they’ve been treated with oseltamivir (Tamiflu).

Viral shedding with seasonal flu is thought not to last more than a week, but the new studies showed that H1N1 continued beyond that—in some cases in the Singapore study, up to 12 days and beyond.

In the Quebec study, a minimum of 8% and as many as 75% were still shedding live virus on day eight after symptoms started. In the Singapore study, where patients were treated with oseltamivir, the average duration of viral shedding was 4.8 days, but that persisted in 20% to 30% of patients for a total eight to 10 days.

The researchers—Dr. Gaston De Serres of the Institut National de Santé Publique du Québec and Laval University in Quebec City, and Dr. David Lye of the Tan Tock Seng Hospital in Singapore—cautioned that the patients may not have been able to transmit the virus because their viral loads weren’t measured. But viral shedding is a necessary condition for infectiousness.

“The question is how long should we isolate people,” Dr. De Serres said.

Because in H1N1 patients, fever generally lasts up to four days and virus was still detected in a substantial proportion of patients, “returning patients to regular activity immediately after fever has subsided appears premature.”

Masks: Australian researchers studied the clinical efficacy of surgical masks and N95 masks, with and without fit testing, in more than 1,900 Chinese health-care workers from 24 hospitals. They found that surgical masks offered no protection, but N95 masks, regardless of whether fit testing was done, had high rates of protective efficacy against proven influenza infection as well as against any proven respiratory viral infection.

However, Dr. Raina MacIntyre of the University of New South Wales who headed the study, recommended fit testing, saying her results with one brand of mask may not be generalizable to other brands.

New drugs: Drug maker Adamas Pharmaceuticals in Emeryville, Calif., reported that its triple combination of amantadine, ribavirin and oseltamivir was “highly synergistic” in vitro and may overcome viral resistance. The company is planning to begin clinical studies of the combination in North America.

Korean, Taiwanese and Japanese researchers reported findings of a phase III study showing that a single dose of I.V. peramivir, a new neuraminidase inhibitor, was comparable to oral oseltamivir in the treatment of acute, uncomplicated flu in outpatients.