MEXICO CITY - Mexico welcomed millions of children back to school Monday with masks, thermometers and globs of hand sanitizer, as scientists estimated that the new strain of swine flu could have sickened 23,000 people before anyone realized it was an epidemic.
At least 61 people have been killed by swine flu around the world, and the World Health Organization has confirmed 4,800 cases, including the first in mainland China. China scrambled Monday to find and quarantine more than 200 people on the infected man's flight from the U.S., though the University of Missouri campus where he had been studying planned no special precautions.
A study published Monday in the journal Science estimated that Mexico alone may have had 23,000 cases by April 23, the day it announced the epidemic. The study estimates that swine flu kills between 0.4 percent and 1.4 percent of its victims, but lead author Neil Ferguson of Imperial College, London, said the data remains incomplete.
"It's very difficult to quantify the human health impact at this stage," he said.
The reopening of kindergartens and primary and middle schools shut since April 24 was the latest step in Mexico's efforts to restore a sense of normality. Businesses, government services, high schools and universities reopened last week.
But six of Mexico's 31 states put off reopening schools for a week because of local rises in the number of cases, and a seventh ordered a one-day delay.
In Mexico City, children lined up outside the Ignacio L. Vallarta public elementary school so teachers could check students for flu symptoms. Some parents worried schools were opening too soon, but many were also relieved after spending two weeks trying to entertain bored children. Officials said any students with symptoms would be sent home.
"It's good that schools are reopening. Our children were getting lazy," said Eugenia Martinez as her 8-year-old son, Edgar, ran around in a white mask and Power Rangers T-shirt. "I think everything is under control."
The federal Education Department said Monday that all 250,000 schools - except some 30,000 in states that did not reopen Monday - had been cleaned and disinfected as 25 million children prepared to return to class.
"It's very important for families to know that the disease is curable; we have enough medicine to treat any cases that arise," Education Secretary Alonso Lujambio Irazabal said. "As soon as we suspect we have a case we are going to offer antivirals to that person, that teacher, that student."
Mexico also is trying to revive its economy after the epidemic pummeled tourism, the country's third-largest source of legal foreign income. Mexico provided details Monday of a 14 billion peso ($1.1 billion) package to help restaurants, hotels and other businesses.
At least 10 commercial banks are involved in the plan, promising three-month reprieves for small businesses with outstanding loans in Mexico City and two hard-hit states. Small businesses in beach resorts and other tourist destinations were promised a six-month grace period.
"We are not looking for magical or spectacular solutions - which would be illusory - only that businesses have the liquidity need to recover from this emergency," Finance Secretary Agustin Carstens said. "Mexico is facing a very complicated year that combines a flu outbreak with one of the most severe global recessions in the last 60 years."
Meanwhile, the number of countries reporting confirmed swine flu cases grew to 30. The United States now has the most confirmed cases - 2,618 - according to the U.S. Centers for Disease Control and Prevention. Mexico has confirmed 2,059 cases. Swine flu has killed 56 people in Mexico, three in the U.S., one in Canada and one in Costa Rica.
The analysis in Science suggests there are many more cases than those confirmed by laboratories - anywhere from 6,000 to 32,000 cases in Mexico as of April 23. The flu has since spread around the world, and the study said it appears to be substantially more contagious than normal, seasonal flu.
Researchers also compared the DNA of the viruses in 23 confirmed cases, and came up with an estimate of Jan. 12 for their earliest common ancestor - presumably when person-to-person transmission began. But with everything that isn't known, they said it could have been anywhere from Nov. 3 to March 2.
The researchers said the 2009 H1N1 flu appears to be about equal in severity to the flu of 1957 and less severe than the deadly 1918 version.