A spokesperson for Novartis AG has revealed the difficulty of producing a vaccine against the H1N1 flu pandemic because the virus yields very little of the antigen needed.

According to Bloomberg.com, Novartis spokesperson Eric Althoff revealed that lab workers are harvesting one dose or less of the component they need from each egg in which the virus is grown, around half or a third of what it takes to provide a typical seasonal flu vaccine.

Most flu vaccines are produced by injecting chicken eggs with an approved version of the virus. The virus is then allowed to grow rapidly within the egg from which it can be extracted as a vaccine. The amount extracted is referred to as the yield.

According to the report the World Health Organisation is trying to produce samples that yield more antigen.