MEXICO CITY - MEXICO City lowered its swine flu alert level from yellow to green on Thursday, and the mayor said 'we can relax' now that there have been no new infections for a week.

Mayor Marcelo Ebrard said the change means the risk of contagion is low, the situation is under control and the images of countless people wearing blue surgical masks in cars, sidewalks, restaurants and theaters can be consigned to history.

City Health Secretary Armando Ahued said nobody has been hospitalised with respiratory infections in the last three days, and no swine flu cases have been confirmed since May 14.

Since the outbreak was declared on April 23, Mexico City has spent 4.5 billion pesos (S$483 million) to buy medicine and antibacterial gel and provide incentives for businesses to shut their doors and clean public spaces.

The Health Department confirmed three more deaths Thursday, raising the toll to 78 nationwide. A total of 4,008 people have been infected across Mexico. The country's confirmed toll has been rising as scientists test a backlog of samples from patients.

Mr Ebrard urged Mexicans to remain vigilant, to maintain sanitary conditions in places where crowds gather such as the subway and schools, and to support a 'culture of health.'

Nearly 80 per cent of those who died were between 20 and 54 years old, according to the Health Department. However, many suffered from underlying medical conditions. Almost 30 per cent were obese or had other metabolical problems, and 13 percent had cardiovascular problems.

New test results from the US Centres of Disease Control show that people in their 60's and older have signs of greater immunity to the new swine flu virus.

Scientists think it's because older people have been exposed to other viruses in the past that are more similar to H1N1 flu than more recent seasonal flus. -- AP