1.1. How do Vaccines Work?

Microorganism-caused infectious diseases have been one of the most challenging problems in human health. The severe ones were so feared in the past that people have regarded them as divine punishments. Smallpox has plagued humanity for at least 300 years, killing more than 300 million people in the 20th century alone. Its eradication in 1980 is one of medicine’s greatest achievements.

In general, vaccination is a “process that safely induces an immune response that confers protection (not developing the disease) against infection and/or disease on subsequent exposure to a pathogen“. A vaccine is, unsurprisingly, a biological product used for vaccination.

Around 100 years after Edward Jenner’s cowpox-based vaccine, Louis Pasteur unveiled the rabies vaccine. This marked the start of a period where vaccines for a lot of diseases were quickly developed - notable examples include diphtheria, pertussis, and typhoid. The second half of the 20th century is often regarded as the golden age of vaccine development, thanks to very big leaps in knowledge about screening and vaccine manufacturing. Nowadays, the World Health Organization estimates that around 2.5 million lives are saved each year by vaccines.

So, why do vaccines work in the first place? The short answer is that they work because humans evolved to have extremely advanced defenses against pathogens (microorganisms that can cause disease). Let’s take an imaginary virus - Virulenta Fictus. Like other viruses, let’s say V. Fictus spreads through the air and infects a person’s lungs first. Viruses replicate by ‘hijacking’ human cells. Their envelopes help them inject their viral genetic material (let’s say DNA) into cells, often through small structures that act as ‘needles.’ The cells then convert the viral DNA into messenger RNA, which then gets turned into proteins, the most common functional structure of biology. DNA instructions injected in the cell also contain the recipe for how to assemble new viruses, from packing viral DNA to assembling the outer shell and (sometimes) exporting it to the outside of the cell, where it can infect other cells (other times, cells just produce viruses until they burst with them). In a sense, viruses turn the cell machinery into a virus factory. You can see how, without any mechanism to limit the spread, a virus can truly be deadly.

Humans, however, have two advanced systems that are ready to combat pathogens. The innate immune system is fast but not very specific or effective - it acts as a first response to infections, and is often enough to prevent most pathogens from spreading. The adaptive immune system, on the other hand, is much slower but highly specific and effective. It produces antibodies, proteins that bind to unique molecules on the pathogen’s surface called antigens. The antibodies, while slow to ‘tune’ during the first time your organism encounters an attacker, are highly specific and effective, as they can prevent the molecules they attach to from performing their functions while acting as beacons for the rest of the immune system. Even better, the adaptive immune system can remember past infections and respond much faster to subsequent ones. This is the mechanism powering vaccines, which essentially tricks the body that there's an attack from a pathogen in order to ultimately make the adaptive immune system produce antibodies against some of the real pathogen’s antigens. When the real pathogen is encountered, its antigens are recognized, and antibodies are quickly produced.