WHO spokesman Gregory Hartl told journalists that such occurrences among a sample of 39,000 people vaccinated were expected and must not put people off being immunised against A(H1N1) swine flu.
"In the case of China we are seeing reports -- we do not have first hand confirmation yet -- that out of 39,000 people vaccinated apparently four people have had adverse events," he said.
"Those adverse events have all been mild, if I'm not mistaken things like muscle cramps and headaches," he added.
"Adverse events are fully to be expected, especially these mild types," he underlined ahead of forthcoming mass swine flu vaccination campaigns in the United States and in Europe.
The vaccines approved so far had been used in seasonal formulation for years and have been among the safest vaccines known to exist, Hartl explained.
"The most important tool we have to fight this pandemic is the vaccine," he added, insisting on its importance for health workers.
China began mass vaccinations against swine flu on September 21 in Beijing, making it one of the first nations to start inoculating its population against the virus.
The most populous nation in the world has been at the forefront of international efforts to produce an A(H1N1) influenza vaccine, with at least five companies receiving government approval for the work.