Vaccines - what's in them and how do they work?

Vaccination against disease-causing pathogens like viruses and bacteria has been saving lives for hundreds of years. Vaccine comes from the Latin vacca (cow) because of Edward Jenner's late 1700s success at preventing often deadly smallpox (variola) by inoculating patients with a related virus, the unsightly but not deadly cowpox (vaccinia). Smallpox was eradicated globally in 1980.

Both boys exposed to smallpox - boy on the right had previous vaccination. Leicester England 1901

So how exactly do vaccines work then? Think back to when you were in school. If you had a big test or project, when did you earn the best grade? When you studied and prepared or when you just winged it? There may have been times you got lucky and did alright without extra prep because you had paid attention in class and had some prior knowledge, but imagine going into a class and being told you have a pop test on something you have never seen or heard about before in your life. How well do you think would you do then? Now imagine that doing poorly on this test decides whether or not you will have permanent disabilities or even if you live or die... You'd probably want to study for the test somehow, right? Think of vaccination like your immune system studying for that pop test rather than just winging it. 

If at first, you don't succeed, try try again. Science backs this up.

We never know exactly when we might get exposed to a new pathogen that could have serious consequences for our life. But for certain pathogens, we've been fortunate enough to develop a training strategy for our immune system - vaccines - so it isn't completely blind-sided if it comes up against them.

Your immune system has many specialized cells that scan your body for anything unusual (like cancerous cells) or foreign (like bacteria or viruses). Vaccines help those cells recognize potential threats and begin the process of making protective antibodies against them. They can contain weakened pathogens, "killed" pathogens or sometimes just a small portion of the pathogen called an antigen.

Vaccines stimulate the adaptive immune response.

Developing the ability to recognize and target a pathogen can take weeks to months after the immune system has its first exposure to the foreign agent. This is why people may say they came down with the flu shortly after they got vaccinated - it means they were exposed to an infectious virus before vaccination was able to help prime their immune system.

How the immune system responds to COVID infection

This shows the lag between exposure and antibody production.

Once they are raised, antibodies will bind to their targets, causing immune cells to slate them for destruction. As a result, a person with antibodies and primed T cells will typically either get a milder form of illness when exposed to a pathogen they've been vaccinated against or in many cases not even get sick at all. However, the reality is that no vaccine is 100% successful because people's immune system responses are variable and sometimes pathogens mutate (check out some success stats here). But the fewer potential hosts a pathogen has in a population, the better shot we have at controlling or eliminating the disease it causes.

There are legitimate reasons why some people cannot be vaccinated against certain diseases - sometimes they are too young or too old, other times they may have a specific allergy to a component of the vaccine or may be immunocompromised due to cancer treatments or organ transplantation. Those people rely on the rest of us with healthy fully functioning immune systems to get vaccinated to protect them by stopping the chain of disease transmission. The amount of people with immunity to a particular disease (either through infection or vaccination) needed for herd immunity to occur varies based on each disease (measles requires 95% of the population to have immunity for unvaccinated folks to be protected for example), but most diseases studied require greater than 80% of the population to have immunity.

Why is it so important to understand this concept? A large-scale study carried out on the blood of Americans last summer through September found that only about 10% of people showed evidence of antibodies against SARS-coV2/COVID. That's a far cry from what would be needed to abate this pandemic. This is why COVID vaccination is critical. Vaccine hesitancy affects us all. 

Quite frankly some of the rhetoric I've heard over the last year has me convinced that polio never would have been eradicated in the US if we'd had the same level of anti-science, anti-public health, anti-government sentiment back in the 1950s and 1960s as we do now. And that's a damn shame.