Switzerland's national drug agency announced Wednesday it was lifting a ban on sales of flu vaccines made by Swiss pharmaceutical giant Novartis after documenting that concerns over impurities were unfounded.

"Swissmedic lifts the suspension of deliveries and use of Novartis flu vaccines (since) the company has managed to document the quality of the batches that have been and will be delivered in Switzerland," the agency said in a statement.

Last week, Switzerland joined Italy, Austria and Germany in freezing the sale and use of Novartis's Fluad and Begripal flu vaccines after white particles were seen in syringes carrying them.

But Swissmedic said Wednesday the company had convinced it that the particles were not signs of impurities but had occurred during the vaccine manufacturing process itself.

"Such (particles) can in effect be created during the manufacturing of the vaccine, but dissolve if the syringe is used in conformity with instructions," the agency said, stressing the syringe should be lightly shaken before use.

"The security and efficiency of the vaccine are in no way weakened by these (particles) present in some syringes," it said, adding that its own labs had tested the vaccines and had confirmed their quality.