"The good news is that so far, everything that we've seen, both here and abroad, shows that the virus has not changed to become more deadly," said Thomas Frieden, the director of the U.S. Centers for Disease Control and Prevention. "That means that although it may affect lots of people, most people will not be severely ill." But he cautioned that "the H1N1 influenza and influenza generally is unpredictable."
A CDC study Thursday showed that most of those U.S. children who have died of the new H1N1 flu were at least five years old, and 67% had high-risk medical conditions, predominantly neurodevelopmental disorders such as epilepsy or cerebral palsy. Some otherwise healthy children who died had bacterial infections, the study found, warning doctors to be on the lookout for them so they could be treated quickly.
Workers practice coughing into their sleeves as a way to try to control the spread of the swine-flu virus, during a meeting at the Maryland Department of Health and Mental Hygiene in Baltimore on Thursday. Health officials welcomed early evidence that one dose of the new H1N1 vaccine may be sufficient to protect some people from infection. If that finding bears out, public health authorities would be able to stretch limited supplies of vaccine to a larger portion of the population.
Their assumption has been that most people would need two doses to build enough immunity to the new virus. World-wide, the new flu has been confirmed to have infected more than 209,438 people, and at least 2,185 have died, according to the World Health Organization.
Chinese vaccine maker Sinovac Biotech Ltd. won approval from China's drug regulator for commercial production of that country's first H1N1 swine-flu vaccine, and said it is effective with a single standard dose. Novartis AG also said that its H1N1 swine-flu vaccine also prompted a strong immune response after a single dose in a pilot trial. The company said that two doses provided better protection, but that the study suggested one dose could be enough to protect adults against swine flu.
U.S. officials are hoping for similar results from their own clinical trials. The Chinese vaccine is similar to shots being tested in the U.S., according to Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases. Initial results are expected sometime in the next two weeks.
The WHO has warned that there isn't enough production capacity to produce shots for the world's entire population. About 25 companies are making H1N1 pandemic vaccines, including Sanofi Aventis SA, GlaxoSmithKline PLC and Novartis. The U.S. has ordered 195 million doses from five companies, and is expected to take delivery of the first 45 million to 52 million doses in mid-October.
The WHO has asked vaccine manufacturers to set aside at least 10% of their vaccine production for the developing world, and is talking with governments, as well as potential nongovernmental donors, about helping poor countries obtain vaccine. The WHO is trying "to do whatever we can to improve access in developing and poor countries to vaccine. This is a high priority for us," said Keiji Fukuda, the WHO's assistant director-general for health security and environment.
So far, though, only two companies have agreed to donate doses for developing nations, according to the WHO: Sanofi with 100 million doses, and GlaxoSmithKline with 50 million. The agency is also trying to obtain a portion of H1N1 vaccine production from other manufacturers at reduced prices, and is providing seed financing and technical assistance to 11 vaccine manufacturers in developing countries.
China is among the countries the WHO is pressing to donate vaccine for the developing world. China says that by the end of the year its vaccine manufacturers will produce enough vaccine for about 65 million people -- or about 5% of its population. That would exceed China's usual seasonal flu-vaccination rate.
Demand for the vaccine is potentially vast in China. The country's work force plays a critical role in world manufacturing, and the global economy could be affected if large numbers of Chinese become ill. Some scientists also fear that if H1N1 were to spread in China's huge population, chances would increase that it could mix with other viruses such as avian influenza and become more deadly.
Mao Qunan, a spokesman for China's Ministry of Health, said experts are discussing how to allocate the vaccine and which groups of people should have priority for immunization, which he said should "start as soon as possible." He said Beijing as yet has no specific plans about providing vaccine outside China, but that "we will do our best" to help other developing countries.
Liu Peicheng, a spokesman for Sinovac, said specific plans for the distribution and use of its vaccine would be decided by the government. "The priority is to meet the domestic demand. The surplus will be provided to other countries," he said. Sinovac is supposed to supply five million doses of the vaccine by Oct. 1, and annual capacity would be 20 million to 30 million doses, the company said.
—Ellen Zhu and Katharina Bart contributed to this article.
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