I think the most important thing we found was the rate of spread and the extent of spread is similar to what has been seen in previous pandemics," Fraser added.
"Our estimates suggest that 23,000 individuals had been infected in Mexico by late April, giving an estimated case fatality ratio of 0.4 percent based on confirmed and suspect deaths reported to that time," they wrote.
"Thus while substantial uncertainty remains, clinical severity appears less than that seen in 1918 but comparable with that seen in 1957."
The 1918 pandemic was the worst of the 20th century, killing anywhere between 25 million and 100 million people, depending on estimates. It was the first appearance of the H1N1 virus. The 1957 pandemic of H2N2 killed an estimated 2 million people globally. Seasonal flu kills about 250,000 to 500,000 people annually with a fatality rate of less than 0.1 percent.
Looking at the pattern in La Gloria, the researchers said it looked as if the virus was transmitted human to human over 14 to 73 generations -- meaning one person infected another, who infected another, up to 73 times.
"Transmissibility is therefore substantially higher than seasonal flu," they wrote.
