Researchers begin to work on producing a swine flu vaccine for human use in Wuhan of Hubei province, China. Some are worried that an expedited approval process could lead to unforseen risks of vaccination.
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Meanwhile, countries in the Southern Hhemisphere, where it is winter, are already seeing a sharp increase in their death tolls from the pandemic flu strain. So far, 137 have died in Argentina, 33 in Chile and 15 in Uruguay. More than 10,000 confirmed cases have been reported in Australia, and health officials there warn that the country could face up to 6,000 deaths this year.
The spike in fatalities, some say, may be a grim preview of what lies ahead for public health efforts in the Northern Hemisphere nations later this year, Gerberding said.
Cases in European countries are also on the rise. Britain has estimated 55,000 new cases in the last week alone, including that of Cherie Blair, the wife of former Prime Minister Tony Blair. More than two dozen people have died from the virus in the United Kingdom, including a 6-year-old girl.
The virus has not taken the summer off in the United States either. More than 50 summer camps in 20 states have sent kids home early or canceled sessions after suspected outbreaks.
The swine flu could spike in the United States around October.
But so far, at least, public health officials are heartened that the virus itself does not appear to have mutated into a more dangerous form.
"Fortunately, there are no signs that the virus itself is becoming more potent," Gerberding said.
She added that Americans can go a long way toward protecting themselves by keeping their hands washed, covering up sneezes and coughs and staying home when sick.
ABC News' Joanna Schaffhausen contributed to this report.
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