As U.S. health care providers await delivery of the first doses of vaccine against pandemic influenza A(H1N1), researchers are finding that some patients with the infection shed live virus a few days longer than commonly occurs with seasonal flu, according to a Canadian study with 100 patients.
The public health implications of the finding aren't clear, Dr. Gaston De Serres said during a press briefing at the annual meeting of the Interscience Conference on Antimicrobial Agents and Chemotherapy.
The results show that it's not enough to isolate people infected with pandemic H1N1 flu for just a couple of days after they become sick or until their fever resolves. People “may be tempted to reduce their time at home [when infected by H1N1], but our results show that would not be wise,” said Dr. De Serres, a medical epidemiologist at the National Public Health Institute of Quebec.
The study focused on 43 patients with symptomatic flu who were culture positive for the pandemic virus. In this group, eight (19%) remained culture positive 8 days after their symptom onset. In contrast, all patients with seasonal flu are routinely culture negative a week after symptom onset. “We can say that H1N1 appears to be shed longer [than seasonal flu] but not much longer,” said Dr. De Serres, who also is professor of epidemiology at Laval University, Quebec. All 43 H1N1 patients in the study were culture negative 10 days after symptom onset.
Another 57 family members of these cases had concurrent flulike symptoms, but all 57 were culture negative the first time they were tested. Adding these 57 to the first 43 produced a total of 100 patients apparently infected with H1N1, of whom 8 were culture positive a week after their illness began, establishing a minimum 8% rate for the persistence of H1N1 shedding beyond a week of infection. The rate might even be a bit greater because all of the family members may not have been infected with H1N1.
Dr. De Serres cautioned that the findings don't mean that all eight patients remained contagious at day 8. Contagion requires more than just shedding live virus; it also requires transmission of an adequate virus dose. The study didn't look at the amount of virus shed on day 8. People who shed live virus “may potentially be contagious; we're not saying they are contagious,” Dr. De Serres said.
Vaccine against pandemic influenza A(H1N1) were on track to reach providers early this month, a Centers for Disease Control and Prevention official said at a meeting of the National Vaccine Advisory Committee.
This timetable applies to the vaccines that received approval from the Food and Drug Administration last month, said Dr. Jay Butler, who heads the H1N1 vaccine implementation program for the CDC in Atlanta. (See related stories on p. 5).
The U.S. government has arranged to purchase 194 million H1N1 vaccine doses, which will be supplied to the U.S. public at no charge for the vaccine (although there will be charges for ancillary materials), said Dr. Robin Robinson from the Biomedical Advanced Research and Development Authority of the Department of Health and Human Services in Washington.
Dr. Anne Schuchat, director of the CDC's National Center for Immunization and Respiratory Diseases, said that during the first 2 weeks of September, 4% of visits to CDC sentinel providers of outpatient or emergency department care were for flu-like illness. That rate is as high as that seen in February 2009, the most recent peak of seasonal influenza in the United States. At press time, the spike of flu cases had been most dramatic in states located in the southeastern United States.
Dr. Schuchat's data also showed that U.S. cases of H1N1 infection never disappeared over the summer, although the reported cases of flu-like illness were down compared with the prior H1N1 peak last April and May. Current assessment of the virus indicates that the strain circulating in early September is not genetically different or any more virulent than the strain that circulated last spring.
Based on recent data on immunogenicity, the new H1N1 vaccine will be administered to people aged 10 and older as a single dose. Children younger than 10 are slated to receive two doses regardless of their immunization history.
Other notable features of the H1N1 infection pattern have been higher than usual infection rates in people aged 5-17 and 18-49, and the unexpected finding that morbidly obese people were among those at increased risk for infection. The highest hospitalization rates for H1N1 infections have been in patients younger than 5, followed by those aged 5-24.
The threat of H1N1 infection in young adults aged 18-24 has created a new target for immunization messages, said Dr. Kris Sheedy, communications director for the CDC's National Center for Immunization and Respiratory Diseases.
Until safety data from broad field use are available, the CDC will need to rely on the fact that the new H1N1 vaccines were made by the same methods that have been applied in past years to produce hundreds of millions of doses of seasonal flu vaccine, Dr. Schuchat said. This record of safety should be balanced against the clear health risk that H1N1 presents, a comparison that should convince most people to get immunized.
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