In order to test whether the flu virus in a sample taken from a patient, matches an influenza vaccine virus, the sample needs to be grown in a cell culture, or an embryonated chicken egg. Growing the virus in cell culture, or eggs, produces a larger volume and concentration of virus than what was in the original sample. The larger volume of flu virus is needed for additional laboratory tests. These eggs had been injected with influenza virus, and were warming in an incubator. The warmth encourages influenza viruses to grow faster than they would at room temperature. The eggs stay in the incubator for two to three days, depending on the influenza viruses they contain. Influenza B viruses take a bit longer to grow with this method, compared to influenza A viruses.
Influenza (flu) viruses change constantly. As a WHO Collaborating Center for Influenza (WHO CC) and the U.S. National Influenza Center, CDC monitors flu activity nationally and globally with other labs, looking for changes in circulating viruses. CDC monitors flu viruses because changes can impact the effectiveness of flu vaccine. When circulating viruses are substantially different from those in the vaccine, vaccine effectiveness can be reduced. If it looks like viruses are starting to change in specific ways (which can impact how well the vaccine works), this can trigger health authorities to recommend different viruses for vaccine production.
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