U.S. Swine Flu Count Nears 1,900; Person-to-Person Transmission Now Common

May 8 -- THURSDAY, May 7 (HealthDay News) -- The number of confirmed or suspected cases of swine flu in the United States was approaching 1,900, federal health officials said Thursday, with most new cases now caused by person-to-person transmission and not some link to Mexico, as was the case when the outbreak began nearly two weeks ago.

"Only about 10 percent of confirmed cases have a travel history for Mexico," which is believed to be the source of the outbreak, Dr. Richard Besser, acting director of the Centers for Disease Control and Prevention, said during an afternoon teleconference. "This indicates ongoing transmission in communities. We would expect that as this [virus] travels more around the country that we are going to see that number go down. While there may have been introduction from travel to Mexico, the spread that's ongoing does not require travel to Mexico," he added.

Besser said there are now 1,823 probable and confirmed cases in 44 states, with most of the infections mild and leading to a quick recovery. The median age of hospitalized individuals with swine flu is 15.

He also said the CDC plans to stop concentrating on reporting numbers of cases and start concentrating on where flu activity is most pronounced in the country. "At some point reporting on individual cases no longer has value from a public health perspective, but knowing where in the country we are seeing large amounts of flu activity does remain important," he added.

Besser also addressed two other significant topics at the teleconference:

• The H1N1 swine flu, thought to be a new virus, shows similarities to strains that first infected people in the United States back in 2005.
• People may be holding "swine flu parties," where individuals knowingly expose themselves to someone with the flu, in the belief that gaining some immunity now may offer greater protection next winter if the swine flu returns in a more virulent form. Besser dismissed the idea as a "big mistake."

On Thursday, the New England Journal of Medicine released early a study by CDC researchers that said 11 cases of infection with a swine flu virus similar to that involved in the current outbreak have been recorded in the United States since 2005. These viral strains were so-called "triple-reassortant" viruses, meaning that -- like the current H1N1 strain -- they contained genes from bird, pig and human viruses.

All but two of the cases involved people who had direct or indirect contact with pigs, but "in another patient, human-to-human transmission was suspected," wrote a team led by Dr. Lyn Finelli of the CDC's Influenza Division. The patients were typically young -- with an average age of 10 years -- and four of the 11 cases were severe enough to require hospitalization, with two needing invasive mechanical ventilation. Four patients were given the antiviral medication Tamiflu and all eventually recovered, according to the report.

Cases of this type of influenza H1 virus appeared to pick up more recently, with eight of the 11 cases being reported by the CDC after June 2007, the researchers noted. The cases were spotted via the CDC's routine "passive" flu surveillance systems and additional but unidentified cases may have occurred, the experts said.

The findings suggest that "all human infections with influenza viruses of animal origin, even those that appear to be clinically mild, warrant a thorough public health investigation to assess the epidemiological risk to humans," the researchers said.

At the teleconference, Besser also discussed reports of so-called swine flu parties.

"Having swine flu H1N1 parties is a big mistake," he said. "This is a new emerging infection and we are learning more each day, but how an individual person will be impacted by the infection is not something we know. It is a big mistake. It is putting individuals and children at risk and CDC does not recommend that people follow that course."

As the outbreak continues, the CDC continues to study various aspects of the health threat to gain a better understanding of the virus, how it is spread, as well as better ways to test for it, Besser said. The research under way includes: finding a rapid diagnostic test; understanding "viral shedding" (how long people can pass the virus to others); determining how the virus is transmitted in households; and learning how well antiviral drugs work.

Testing has found that the swine flu virus remains susceptible to two common antiviral drugs, Tamiflu and Relenza, according to the CDC.

On Tuesday, U.S. health officials said the outbreak of swine flu appears similar to the seasonal flu in its severity, so schools across the nation should remain open and any schools that did close should reopen.

(As of May 7, 2009, 11:00 AM ET)