The Health Effects of Vaccines vs. Antibiotics

Antibiotics and vaccines are in some ways opposites.

Answer by Suzanne Sadedin, Ph.D. Evolutionary Biology, Monash University

What are the connections between the responses of nature to the ways humans have used vaccines and the ways they have used antibiotics? originally appeared on Quora, the place to gain and share knowledge, empowering people to learn from others and better understand the world. You can follow Quora on Twitter, Facebook, and Google Plus.

Antibiotics and vaccines are in some ways opposites. Antibiotics kill indiscriminately, whereas vaccines are highly targeted. Antibiotics are used to treat severe infection, whereas vaccines prevent infections from ever becoming established. And antibiotics are based on defenses that evolved in microbes, to protect them from bacteria; they are not a natural defense for us, and our bodies are not adapted to cope well with them. Vaccines, in contrast, simply invoke the human body’s natural long-term defense systems, and are therefore far less invasive.

How does nature respond to antibiotics?

Antibiotics create selective pressure on a wide range of bacteria wherever they are used. Humans naturally host large numbers of bacteria. These are essential to our health and killing them off repeatedly risks many long-term health problems, including immune disorders, damage to the gut, and increased vulnerability to infections.

In addition, when you use antibiotics, you put all these bacteria under selection to resist antibiotics. Worse, since antibiotics are often excreted intact, low concentrations of antibiotics are now found in water supplies everywhere, creating a perfect environment for bacteria in general to evolve resistance. Bacteria exchange genetic material with other strains of bacteria, especially via plasmids. So once a mechanism to resist an antibiotic evolves in one strain, we can expect it to spread to many different strains — including those that cause severe disease.

In recent years, plasmids have been found in Africa that confer resistance both to multiple antibiotics and a common disinfectant. Salmonella bacteria with these plasmids are now a major cause of mortality among African children. As one expert commented: “It’s almost designed by nature to be the perfect solution to man’s attempt to treat with antibiotics.”

But resistance isn’t the only effect. Antibiotic treatments commonly kill almost all the bacteria, leaving only a few survivors. This is called an evolutionary bottleneck and its result is genetic drift. That is, the few survivors, apart from being resistant, may be quite random, so the genetic composition of the population can change rapidly, causing it to evolve in new and unpredictable directions — including increased pathogenicity.

So: the long term expected effect of antibiotics is that (1) antibiotics create resistant strains of pathogenic bacteria (2) antibiotics can create strains that are either more or less severe than the parent strain (3) by wiping out natural biota, antibiotics put us at risk of long-term health problems.

How does nature respond to vaccines?

Vaccines create selective pressure only on the specific infection they target. Due to herd immunity, even some people who cannot be vaccinated receive some protection from widespread use of vaccines (provided everybody else does the right thing). In addition, since vaccines prevent the target replicating at all in the host, they create no bottleneck. In fact, as hosts become more rare, the pathogen is under selection to lie low and avoid harming its host, because it may be a long time before it can spread to a new host. Also, the vaccine does no harm to normal human biota, and therefore does not significantly disrupt the gut, immune system or any other part of the body.

So: the long-term expected effect of vaccines is that (1) vaccines do not affect the evolution of non-targeted strains (2) vaccines cause the targeted strain to evolve to become less severe (3) vaccines are one of the safest medical interventions.

Comparison of long term expected effects on…

…evolution of resistant strains: Antibiotics — yes, including non-targeted strains. Vaccines — do not create resistance.

…evolution of disease severity: Antibiotics — unpredictable. Vaccines — make diseases less severe.

…health of treated people: Antibiotics — harmful (but hopefully less so than the infection they treated). Vaccines — harmless.

…health of untreated people: Antibiotics — none or harmful. Vaccines — beneficial.

Both antibiotics and vaccines are wonderful inventions. They have saved countless lives and spared humanity immense misery. I don’t think current generations can even imagine what life was like without them. Vaccines are vastly preferable to antibiotics; we should use vaccines freely, and antibiotics only when necessary.