CDC: H1N1 spreads about as readily as seasonal flu

May 20, 2009 (CIDRAP News) – A federal influenza expert said today that the novel H1N1 flu virus seems to spread at about the same rate as seasonal flu viruses do, even though much of the population is believed to have little or no immunity to the virus.

At a news briefing, Dr. Daniel Jernigan, deputy director of the influenza division at the Centers for Disease Control and Prevention (CDC), said it appears that the new virus has about the same attack rate as seasonal flu. The attack rate is the proportion of people who come down with the illness after being exposed to someone who has it.

Early in Mexico's H1N1 epidemic, some studies suggested that each person sick with the new virus was spreading it to many others, Jernigan said.

However, "The more we look at it here in the United States, what we're seeing is the attack rates really coming in at about what we would see with seasonal influenza," Jernigan said.

From the studies so far, the transmission both within households and within communities seems to be about the same as with seasonal flu, he said.

"From a policy standpoint, we expect this to be spreading the same as we would see with seasonal influenza, but again, remember that a larger portion of the population may have absolutely no immunity or any protection for this one, which is different than what would happen through normal seasonal influenza," Jernigan added.

Jernigan also commented on the possibility that older people have some protection against the new virus because of exposure to H1N1 viruses decades ago. In a report earlier this week, a World Health Organization advisory group said older adults were shown to have neutralizing antibodies to the new virus. Jernigan said this statement in the report was probably based in part on CDC serology studies.

He said older people most likely were exposed to H1N1 viruses—distant cousins of the new virus—before 1957, "and there's a possibility that having exposure to that virus many years ago may allow you to have some reaction to the new H1N1 that's now circulating."

Jernigan said it's not yet known just how much protection older people may have. He added that the CDC will be reporting in more detail on the topic in an upcoming issue of Morbidity and Mortality Weekly Report.

In other comments, Jernigan said the production of this year's seasonal flu vaccine is nearly complete. He added that the CDC still hopes to have candidate H1N1 viruses ready to send to manufacturers at the end of this month for use in the possible production of H1N1 vaccines.

See also:

Transcript of May 20 CDC news briefing
http://www.cdc.gov/media/transcripts/2009/t090520.htm

May 6 CIDRAP News story "Fewer senior swine flu cases may hint at protection"

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