Mexican officials confirmed on Wednesday the first mutation of the A(H1N1) flu virus in a girl who survived the infection.
Health Minister Jose Angel Cordova told Mexican journalists that the case was the first confirmed mutation of the swine flu virus, though there were 423 other suspected cases.
He said the girl was treated two months ago at a hospital in Mexico City for a respiratory illness and then returned with a case of severe pneumonia, from which she recovered.
Cordova called on anybody with risk factors that could make them more susceptible to the virus to be vaccinated against it, warning that "these viruses can mutate at any time" with serious consequences.
Officials say 1,088 people have died in Mexico from the virus commonly referred to as the swine flu since the pandemic first emerged in the country in April 2009.
Nearly 16,000 people have died worldwide from the strain after it spread into 212 countries and territories.
The World Health Organisation said last month it was too early to declare that the peak of the global flu pandemic had passed as infections were still rising in regions such as western Africa.