US health officials said Saturday that swine flu was continuing to spread in the country, with the number of confirmed cases rising to 2,254, with 104 people hospitalized.
The cases were confirmed in 44 of the nation's 50 states and the capital Washington but health authorities were now focusing on the characteristics of the new virus and on developing a vaccine, said Anne Schuchat of the US Centers for Disease Control and Prevention (CDC).
On Friday, there had been 1,639 confirmed A(H1N1) influenza cases in the United States, which has recorded two deaths so far in patients with underlying health issues.
Most of the hospitalized patients, Schuchat said, also had other health problems. US health officials have said they expect to find confirmed cases in all 50 states.
Mexico is at the epicenter of the global A(H1N1) epidemic, with 48 deaths recorded there compared to two US deaths, but the United States has overtaken its southern neighbor to become the country with the most number of patients.
"I know that in every state, it's really easy to focus on the numbers, but I think right now, the numbers don't tell us as much as the trends," said Schuchat, CDC's interim deputy director for science and public health.
"Our assessment is that transmission here in the US is ongoing, that this is a very transmissible virus, similar to the seasonal influenza viruses."
Despite almost 3,000 probable and confirmed cases now in the United States, Schuchat told reporters that "fortunately, the severity of illness that we are seeing at this point doesn't look as terrible as a category five kind of pandemic or the severity of impact that some had feared."
Noting that the "influenza virus is unpredictable, it can change over time," she said that health officials should focus on how the virus is spreading, developments in the southern hemisphere, which is just starting its flu season, and anticipating the virus's impact during the northern hemisphere's fall.
US President Barack Obama warned Friday that the autumn and winter flu season later in the year could be "even worse" and see cases spike again.
"So a lot of our emphasis here in the US is still understanding the epidemiology, transmission, severity and viral characteristics, but also working with partners internationally to really prepare and evaluate issues in their countries," said Schuchat.
The potentially deadly virus is a hybrid drawn from strains found in pigs, birds and humans. Meanwhile, some countries are still affected by the H5N1 bird flu strain, which first emerged in Asia in 2003 and has since caused nearly 250 deaths, according to WHO figures.
"So we do think this is an unusual and difficult circumstance to have circulation of this H1N1, the regular seasonal flu viruses and in certain countries, the H5N1," Schuchat said.
Scientists fear that a mutation of the bird flu virus resulting in a strain easily transmitted among humans could create a pandemic, potentially affecting up to one fifth of the world's population.
The swine flu outbreak has now been reported in 29 countries with a total of 3,440 cases before the latest US numbers, according to the World Health Organization.