Vaccines against microorganisms and viruses would not have been possible without someone coming up with a simple way of growing these infectious agents on an industrial scale that allows them to be harvested in large quantities. Step forward, Ernest William Goodpasture, an American pathologist and physician, who in 1931 invented methods of growing viruses and rickettsia – the gram-negative bacteria that cause typhus – in chicken embryos and fertilised chicken eggs. This approach led to the development of vaccines against influenza, chicken pox, smallpox, yellow fever, typhus and many other diseases.
Goodpasture’s work was subsequently expanded on during the 1940s, especially by the US military which developed the first approved inactivated vaccines for influenza, which were used in the Second World War. Chicken eggs have continued to be used to incubate seasonal and pandemic influenza vaccines ever since. The virus is introduced into the allantoic fluid of the fertilised egg, which bathes the embryo and yolk sac, where it replicates in the membrane surrounding the fluid. After about three days, the virus-containing fluid is harvested from each egg and the next stage of the vaccine-manufacturing process begins – purifying and inactivating the live virus and preparing it for use as a safe vaccine.
Eggs are still important for vaccine production but the manufacturing process is vulnerable to supply problems caused by anything from bad weather to poultry disease. With influenza vaccines, some strains of the virus grow less readily in chicken eggs than others, making pandemic planning difficult. This is why some vaccine manufacturers are looking at alternatives to working with chickens’ eggs.