But scientists at the Harvard School of Public Health (HSPH) in Boston, Massachusetts have discovered that the deadly 1918 strain was not as contagious as previously thought.

"We found that a case probably infected two to four other people and if we were to try to stop a similar pandemic we would need to block half to three-quarters of transmissions either through vaccination or other measures like isolation," HSPH researcher Christina Mills said in an interview.

"It is an achievable goal but right now we don't have an adequate supply of vaccines to accomplish it," she added.

If a pandemic strain of flu did occur, a vaccine would have to be designed and developed to work specifically against it, which could take months.

The seriousness of the situation became evident earlier this year when half the United States' flu vaccine supply was wiped out because of contamination problems at a manufacturing plant.

Mills and her colleagues analysed historic epidemic data from 45 cities in the United States to assess how transmissible the 1918 strain was. Their research is reported in the science journal Nature.

Although the 1918 flu was no more contagious than many other diseases, it killed many infected people.

"We could control a potential pandemic with technology and a fast enough response," said Marc Lipsitch an associate professor at HSPH and a co-author of the report.

"There is even more reason to make an effort to improve our technology so we are able to develop vaccines more quickly and get antivirals to people who need them more quickly."

He added that a key component to any prevention effort would be to stockpile antiviral medicines in Asia where scientists fear the next pandemic could emerge if the H5N1 strain of bird flu jumps to humans.