GENEVA (AP) -- Vaccine is the best tool against swine flu despite reports of a few minor side effects from the initial campaign in China, the World Health Organization said Monday.

Four out of 39,000 people vaccinated against H1N1 in China have had side effects such as muscle cramps and headache, WHO spokesman Gregory Hartl said.

"Adverse events are fully to be expected, especially these mild types," Hartl said, adding that this was particularly true in cases where very large numbers of people are being vaccinated.

The vaccination campaign will soon move to Australia, the United States and parts of Europe, he said, encouraging people - especially health care workers - to be vaccinated.

"The vaccine is the single most important tool that we have against influenza," Hartl said. "For certain groups such as health care workers, it's doubly important to get vaccinated because health care workers have the ability to protect both themselves and to protect others by getting vaccinated."

The U.S. government will be tracking possible side effects when mass flu vaccinations begin this month in hopes of quickly detecting any rare problems that are actually caused by the vaccine and not pure coincidence.

U.S. health authorities hope to give swine flu vaccinations to more than half the 300 million-plus population in just a few months.

The last mass inoculations in the United States against a different swine flu, in 1976, were marred by suggestions of a link in some cases to a rare paralyzing condition, Guillain-Barre syndrome.

Scientists never could prove that the vaccine really was linked to the syndrome, an often reversible but sometimes fatal paralysis.

Copyright 2009 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.