While I was sitting on the bus half-asleep a week or two ago, I heard (a.k.a. eavesdropped because I did not have any music and I was bored) one fellow talk to his female companion about his favourite co-op experience so far, which had to do something with preparation of vaccines. Much to my surprise, he boasted the claim that “more chicken eggs are used to make the flu vaccine each year than used in food,” which you can imagine is a very bold claim. While I struggle to find an exact statistic to compare to for both (I feel like we do use a LOT of eggs every day), one vaccine production location in Dresden, Germany is reported to use up to 360 thousand eggs per day at the height of flu vaccine-production season. That’s a lot of lil’ chicken embryos. Since it is coming up to the season of flu and vaccines, it seems appropriate to discuss the chemistry and history of vaccines.
Yes, I did say chicken embryos there. Odd as it may sound, the portable little wombs serve as a wonderful breeding ground for the viruses that are to be protected against—and have been used since the 1950s for doing so. Now maybe it makes sense why the nurse asks you if you are allergic to eggs prior to injection. When all is said and done, there is typically a ratio of about one egg needed per vaccine produced—a rather daunting thought when pandemic outbreaks requiring new vaccines are considered. Getting that many healthy, fertilized eggs on short notice can be difficult, if not impossible. For this reason, faster manufacturing processes that utilize other methods (including DNA and cell-culturing) are being researched and currently are in the process of clinical trials.
The current standard manufacturing process begins when each egg is inoculated with one of multiple chosen strands of the flu virus; the exact strains vary each year and are chosen by the World Health Organization based upon surveillance of prominence of the virus. Typically, the concoction of choice varies based on the hemisphere of destination for the vaccines. Following this inoculation, the viruses are incubated, and allowed to breed and multiply for three days prior the lovely virus-filled juices being sucked out of the eggs (not unlike how you do when decorating eggs at Easter, although surprisingly enough it’s not recommended to do so orally for personal health reasons).
However, this flu-ridden liquid is not what we end up injecting into people, thankfully, as it would almost certainly actually cause the flu and prove every “anti-vaxx”-ers wildest dreams to be true. The serum is still subjected to centrifugation and chemical treatment to inactivate the virus to what should be safe for human injection, whilst still containing all of the biological information our bodies need to prepare ourselves against possible infections.
Three to four different strains are typically prepared separately to be later mixed for the single all-purpose vaccine you will receive, so you thankfully don’t have to have nightmares of three consecutive injections. In trivalent vaccines, three strains are chosen: two Type-A, and one Type-B strain. While there is also a Type-C strain of the flu, it is rather uncommon and typically only causes mild cases in the very young.
Typically, the two Type-A strains (or “serotypes”, the name given to distinct variations within a species of viruses) that are chosen are strains of H1N1 (the source of the Spanish Flu in 1918 and the Swine Flu pandemic in 2009) and H3N2 (the source of the Hong Kong Flu of 1968).
The HxNx name refers to the number of antigens that they each express. The H antigen refers to the glycoprotein hemaggluttinin, which facilitates the biding and entry of the virus into the target cell. Neutralizing antibodies produced in response to this H antigen can take one of two actions to inhibit both actions of the hemaggluttinin: prevent attachment of the virus onto the target cell or prevent entry into the cell if attachment occurs.
The N antigen refers to the gycloprotein neuraminidase, which causes the cleaving of gyclosidic linkages of acid groups within the host cell; this allows for the virus to be released from the host cell and for replication to occur. As a result, another most effective ways to treat the flu once caught is with neuriminidase inhibitors, so the virus cannot replicate.
Some flu shots are actually quadrivalent, and contain a second plausible Type-B strain in addition to the three strains already mentioned already mentioned. Because of a combination of factors—including slow mutation rates, which have lead to such little diversity that there aren’t even different named serotypes for Type-B strains—these sort of viruses are not at risk of causing pandemics the same way Type-A viruses are.
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