People in Mexico have been wearing masks as a precaution
A third of the world's population could be infected with swine flu, expert projections suggest.
Researchers say swine flu has "full pandemic potential", spreading readily between people and is likely to go global in the next six to nine months.
Although one in three who come in contact will likely become infected, the Imperial College London team declined to estimate the death toll.
The study based on Mexico's experience is published in the journal Science.
The number of laboratory-confirmed swine flu cases has reached 5,251 in some 30 countries around the world, with 61 having died from the disease, the World Health Organization has confirmed.
Working in collaboration with the WHO and public health agencies in Mexico, the researchers assessed the Mexico epidemic using data to the end of April and taking into account factors like international spread and viral genetic diversity.
Lead researcher Professor Neil Ferguson said it was too early to say whether the virus will cause deaths on a massive scale, or prove little more lethal than normal seasonal flu.
His "fast and dirty" analysis of Mexico's swine flu outbreak suggests that the H1N1 virus is about as dangerous as the virus behind a 1957 pandemic that killed 2 million people worldwide.
But it's not nearly as lethal as the bug that caused the 1918 Spanish flu pandemic, which caused an estimated 50 million deaths in 1918.
Its full impact on the UK is not likely to be known until the annual flu season in the autumn and winter, when a "really major epidemic" can be expected in the northern hemisphere, says Professor Ferguson.
Prof Ferguson, who sits on the World Health Organisation's emergency committee for the outbreak, told the BBC Radio 4 Today programme: "This virus really does have full pandemic potential. It is likely to spread around the world in the next six to nine months and when it does so it will affect about one-third of the world's population.
"To put that into context, normal seasonal flu every year probably affects around 10% of the world's population every year, so we are heading for a flu season which is perhaps three times worse than usual - not allowing for whether this virus is more severe than normal seasonal flu viruses."
His study suggests swine flu could kill four in every 1,000 infected people.
Professor Ferguson said his findings confirmed that decisions must be taken swiftly on vaccine production.
"We really need to be prepared, particularly for the autumn. At the moment, the virus is not spreading fast in the northern hemisphere, because we are outside the normal flu season, but come the autumn it is likely to cause a really major epidemic.
"One of the key decisions which has to be made this week by the world community is how much do we switch over current vaccine production for seasonal flu to make a vaccine against this particular virus? I think those decisions need to be made quickly."