Pictures: ROGER ALLEN

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THE virus is then deactivated using formaldehyde and the solution is purified with a centrifuge. The solution remaining contains the virus antigen. This triggers a human immune response to the infection without causing illness.

Seqirus produces four strains of the virus, identified by the World Health Organisation in February, and mixes them to form a quadrivalent vaccine (below) to protect patients against the four different strains of the flu virus

THE eggs are washed, sorted and incubated in a series of 37 rooms, each housing 172,000 fertilised eggs. They are kept in strictly controlled conditions for 11 days in order to create the perfect environment for a virus to be reproduced. The eggs are stored on racks that change their angle of tilt every hour and stay at a precise temperature of 37.5c (99.5f) and 65 per cent humidity — the ideal conditions to develop their embryos

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THE eggs are then ready for the production line, which scans 132 eggs at a time and rejects those that are infertile — 45,000 a day — before being jabbed with a needle with 0.2 ml of ‘working seed’, a solution that contains the active flu virus, which has been cultured in laboratories. After this, the influenza virus begins multiplying within the shell

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IMPERFECTIONS are prevented throughout the process by randomly examining the eggs in a process called ‘candling’, in which experts use ultraviolet probes to check whether or not an egg is still fertile. Infertile eggs are discarded as they are discovered

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AFTER being incubated for 72 hours, the eggs are ‘blast chilled’ to make sure that the embryo stops growing. The virus-filled eggs now join a harvest line, where a huge machine uses a blade to slice off the top of each eggshell. Probes extract some of the clear liquid from inside the egg, known as allantoic fluid — which now contains thousands of copies of the influenza virus

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THE complete vaccine solution is collected in giant 500 litre sacks. This sterile machine then fills syringes at high speeds

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THE solution is measured and put into 0.5 ml syringes before the vaccine batches are thoroughly checked by workers at the production facility

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THE final product is distributed to doctors and pharmacists across the country