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.

"There's no longer any need" to wear masks, Ebrard said. "Now you can come to the city without any risk."

City Health Secretary Armando Ahued said nobody has been hospitalized with respiratory infections in the last three days, and no swine flu cases have been confirmed since May 14. "We are seeing a 96.1 percent drop in cases, and that's why we are dropping the alert level to green today," Ahued said.

Since the outbreak was declared on April 23, Mexico City has spent 4.5 billion pesos ($334 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.

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."

"The big lesson is that we understand what the scientists have been saying for a decade: that we have to be prepared for any possible virus that can appear," the mayor said.

Nearly 80 percent 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 percent were obese or had other metabolical problems, and 13 percent had cardiovascular problems.

New test results from the U.S. Centers 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 swine flu than more recent seasonal flus.