Dödligt virus från Kina orsakar panik (SARS-CoV-2, COVID-19)

Second difference: number of pathogens

Another key difference between antibiotics and vaccines is when they are used and how many pathogens are around. Antibiotics are used to treat an already established infection when millions of pathogens are already in the body. But vaccines are used as prevention. The antibodies they create can act at the very beginning of an infection when pathogen numbers are low. This has important consequences, as resistance is a numbers game. A resistance mutation is unlikely to occur during the replication of a few pathogens, but the chances increase as more pathogens are present.

This does not mean resistance to vaccines never evolves: a good example is flu. Thanks to its high mutation rate, the flu virus can quickly accumulate enough mutations that antibodies may not recognise it anymore – a process called “antigenic drift”. This explains in part why the flu vaccine has to be changed each year.
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But we should still be careful. As mentioned earlier, numbers matter when it comes to resistance. The more viruses that are around – as in a rapidly growing pandemic – the more likely it is one may hit the jackpot and develop mutations resulting in a significant impact on vaccine efficacy. If that’s the case, a new version of the vaccine may be necessary to create antibodies against these mutated viruses. This is also why trying to keep infection numbers low through prevention and contact-tracing is vital to keeping vaccines working for as long as possible.