Swine flu infections in Mexico and the US have peaked but could return with a vengeance in the autumn, senior health officials said yesterday.

The two nations hit hardest by the influenza A (H1N1) strain said they were cautiously optimistic that the worst was over for their populations. Cases were confirmed in two further countries and the World Health Organisation agreed that North American outbreaks may have peaked.

Health authorities gave warning, however, that the virus could return when the winter flu season begins, similar to the Spanish flu that killed 50 million people in 1918-19.

The continued spread of swine flu meant that a pandemic remained imminent. Gregory Härtl, a WHO spokesman, said: “The Spanish flu showed a surge in the spring and disappeared in the summer, only to return in the autumn with a vengeance. We cannot lower our guard.”

The first case of swine flu being passed from humans to animals was reported in Canada, where a farm worker who had been in Mexico infected a herd of pigs in Alberta.

The WHO said that such incidents had been expected and would not pose a large risk to humans. There was no case for import bans or culls because cooking pork killed the virus.

José Ángel Córdova, the Mexican Health Minister, said that infections appeared to have peaked between April 23 and 28. “The evolution of the epidemic is in its declining phase,” he said. The number of confirmed cases in Mexico is 506 and the number of confirmed deaths is still 19.

The US Centres for Disease Control and Prevention (CDC) said that swine flu had been identified in 30 states, with 226 confirmed cases. It added there were signs that the virus was mild and that the situation was improving.

Richard Besser, the acting director of the CDC, said: “The good news is when we look at this virus right now, we’re not seeing some of the things in the virus that have been associated in the past with more severe flu. Each day we’re getting more information . . . and we’re starting to see encouraging signs.”

Kathleen Sibelius, the US Health and Human Services Secretary, said there were concerns that the virus could return. “We really don’t know what’s going to happen when real flu season hits,” she said.

Mr Härtl said that it has no current plans to convene a meeting of its emergency committee to discuss moving the virus to a phase six. He added that it was possible “that this current round of activity has peaked, but we are only ten days into this outbreak so we must wait and see”.

He added: “There is a high possibility that this virus period will come back, especially in colder periods. We are entering the summer months when normally influenza activity is low.”

Alan Johnson, the Health Secretary, said that the virus had been contained in Britain but he expected more infections later in the year. “Our evidence from all previous pandemics is that you get two phases,” he said. “You get a first wave that is often very mild and then you get a much more serious wave that comes along in the autumn and the winter,” he told The Andrew Marr Show on BBC One.

Three British cases of swine flu were confirmed yesterday, bringing the total to 18. South Hampstead High School, an independent girls’ school in London, became the third to close during the outbreak after a 14-year-old pupil caught the virus from somebody who had been to Mexico. An 11-year-old girl from South London and a man from Ayrshire became ill after travelling to the US.

The Health Protection Agency said that 716 cases were under investigation though only a small proportion were likely to be positive.

The WHO said that the worldwide count of confirmed cases was 787, including 20 deaths. This did not include updated figures released by several countries yesterday.

Spain has confirmed 40 infections, making it the worst affected country in Europe. All but two were in people who had travelled to Mexico. Italy and Colombia confirmed their first cases, making the total number of affected countries 19. It was the first recorded case in South America.

Egyptian riot police clashed with stone-throwing pig farmers, angered by the country’s decision to slaughter their animals despite assurances from the WHO that it was unneccesary.

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