“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.

The death earlier this week of a woman in Texas, the first U.S. resident to die from the swine flu, “reminds us that influenza can be a very serious infection, and it’s one we need to continue to take very seriously,” Besser said Wednesday.

According to published reports, Texas health officials said the death of 33-year-old schoolteacher Judy Trunnell was apparently not directly caused by H1N1 swine flu, noting that she also had unspecified “chronic underlying conditions.”

Last week, a 23-month-old boy from Mexico, who also had underlying health problems, died from the swine flu illness in a Houston hospital. He was the first fatality in the United States from the current swine flu outbreak.

On Thursday, the World Health Organization (WHO) said that as many as 2 billion people around the globe could become infected by swine flu, if the current outbreak turns into a pandemic.

A published report Wednesday said the WHO could decide as early as next week to call for international production of an A H1N1 swine flu vaccine. The head of the agency’s initiative for vaccine research told the Canadian Press that such a decision could force some vaccine manufacturers to make some lots that do not include a vaccine against influenza B viruses.

The CP also said that an expert panel will meet May 14 to review the available science on the swine flu and advise WHO Director General Margaret Chan on whether to call on vaccine manufacturers to make a vaccine to protect against the new H1N1 virus.

On Thursday, the World Health Organization was reporting almost 2,400 confirmed cases of swine flu in 24 countries, with Canada, Spain and the United Kingdom having the most cases outside of Mexico and the United States.

There were 1,112 laboratory-confirmed cases in Mexico, the source of the outbreak, including 42 deaths. The United States had reported 896 confirmed cases, including two deaths, the WHO said.

Meanwhile in Mexico City, the government has now allowed all businesses to reopen, the Associated Press reported Wednesday. This includes sports arenas, movie theaters and restaurants. However, businesses must screen for those who might be sick and surgical masks will still be mandatory for workers and customers, the AP said.

High schools and universities reopened Thursday after a two-week closure intended to limit infections. Younger children returned to school on Monday, the news service said.

More information

For more on swine flu, visit the U.S. Centers for Disease Control and Prevention.
SOURCES: May 7, 2009, teleconference with Richard Besser, M.D., acting director of the U.S. Centers for Disease Control and Prevention; May 6, 2009, news release, U.S. Food and Drug Administration; May 5, 2009, teleconference with Kathleen Sebelius, secretary, U.S. Department of Health & Human Services, and Richard Besser, M.D., acting director of the U.S. Centers for Disease Control and Prevention; Associated Press; Canadian Press

By Steven Reinberg and E. J. Mundell
HealthDay ReportersLast Updated: May 07, 2009