Flu vaccine is usually grown in fertilized chicken eggs (see figure 2). In February preceding each fall's flu season (in the Northern hemisphere), three strains of flu are selected and chicken eggs inoculated.
As of November 2007, both the conventional injection and the nasal spray are manufactured using chicken eggs. The European Union has also approved Optaflu, a vaccine produced by Novartis using vats of animal cells. This technique is expected to be more scalable and avoid problems with eggs, such as allergic reactions and incompatibility with strains that affect avians like chickens. A DNA-based vaccination, which is hoped to be even faster to manufacture, is currently in clinical trials, but has not yet been proven safe and effective. Research continues into the idea of a "universal" influenza vaccine (but no vaccine candidates have been announced) which would not need to be tailored to work on particular strains, but would be effective against a broad variety of influenza viruses.
Figure 2. Researchers infect eggs and sort through eggs used for the cultivation of flu vaccine. Vaccines are made the same way they've been made for half a century, relying on tens of millions of chicken eggs to grow the virus.
In a 2007 report, the current global capacity of approximately 826 million seasonal influenza vaccine doses (inactivated and live) was double the current production of 413 million doses. In an aggressive scenario of producing pandemic influenza vaccines by 2013, only 2.8 billion courses could be produced in a six-month time frame. If all high- and upper-middle-income countries sought vaccines for their entire populations in a pandemic, nearly 2 billion courses would be required. If China pursued this goal as well, more than 3 billion courses would be required to serve these populations. Vaccine research and development is ongoing to identify novel vaccine approaches that could produce much greater quantities of vaccine at a price that is affordable to the global population.
An effective method of vaccine generation that bypasses the need for eggs is the construction of "influenza virus-like particle (VLP)". VLP is a non-egg, non-mammalian cell culture-based vaccine, purified from the supernatants of Spodoptera frugiperda Sf9 insect cells following infection of baculovirus vectors encoding an expression cassette made up of only three influenza virus structural proteins, hemagglutinin (HA), neuraminidase (NA), and matrix (M1) VLPs elicit antibodies that recognize a broader panel of antigenically distinct viral isolates compared to other vaccines in the hemagglutination-inhibition (HAI) assay.