Vaccines have been commonly described as “the single most life-saving medical innovation ever in the history of medicine”. The World Health Organisation calls vaccination “one of the biggest success stories of modern medicine”. It is estimated that vaccinations prevent between 2 and 3 million deaths each year. This is no mean feat. It took quite a while to materialise. The use of vaccination in the world started with the boast of a milkmaid from Bristol, England, in the late 1770s who said “I shall never have smallpox for I have had cowpox. I shall never have an ugly pockmarked face”. What she described became the core principle behind what we now know as vaccination: “protection by exposure to a weak infection.”

So what do vaccines do and how do they protect us from bugs which we can’t even see? Before answering this question let’s try to understand the immune system.

All animals are born with a system that protects them from infections and forms a defensive shield against intruders like viruses, bacteria and fungi. This system is acquired during pregnancy, slowly starts its function at birth and is called the immune system. The system comes programmed with free “basic software”, providing important “shields” to fight a wide range of infection threats. These free versions enable a generically broad but basic level of protection and are grouped together to be called “innate immunity”. It consists of highly qualified and technically capable cells, broadly termed as “white blood cells”, which are freely available in the blood in very high concentrations. These cells directly attack intruder disease-causing organisms, kill them by physical attack or eat them up. Various types of white blood cells exist, each having a different way of attack. In the medical world these types are known as neutrophils, eosinophils, basophils, mast cells and monocytes.

The immune system comes with an additional layer of protection which is its intelligence and the ability to learn new skills and that too very rapidly. This “adaptive immunity” gives the body the ability to think and produce novel ways of combat based on intelligence gathered by its patrols. The core functionality is provided by yet another type of white cells called lymphocytes.

Any infection entering the human body is challenged by the innate immunity. More often than not, it succeeds in getting rid of the invaders. Some micro-organisms prove to be heavy weights, physically overcome innate cells and require extraordinary measures to stop their spread. Special immune cells are on constant look out for these scenarios, promptly report these and call for help as soon as they recognise a failure. Once this message is received, lymphocytes quickly calculate the method and type of cells needed to control the infection. The system checks its inventory to see if these are available from a previous instance and if not, orders the body to start its production. Bespoke, sharp and “specific to the intruder” cells produced are of two types: one directly wrestles with the infectious agents (T-lymphocytes) and kills them; the other type (B-lymphocytes) produces antibodies that attach on specific parts of the infection and overpower them and feed them to scavenger cells. This last defence line is very effective but if produced from scratch it takes days to kick off and up to two weeks to come into full swing. The good thing is that once produced, these cells stay in the body for the rest of one’s life.

This lag time is when the body is most vulnerable and the micro-organisms are free to roam around unchecked. Most of the infections cause little or no harm and simply roam around without causing any grief. Others cause damage to varying degrees, impairing the normal state of the body and in severe cases, interrupting and modifying vital functions. It’s the damage done during this phase that defines the nature of the illness in an individual. A young and healthy body produces quick and strong immune responses overcoming the infection before any serious damage, sometimes even before any symptoms develop. The response is slower and weaker in older, weaker people. In extreme cases, people may not mount any response letting the infection cause severe damage, even death. In Covid-19, elderly people, people with diabetes, cardiac disease and some other risks fall into this category.

A vaccine against a particular micro-organism provides our adaptive immunity intelligence about its structure and genes.

A vaccine against a particular micro-organism provides our adaptive immunity intelligence about its structure and genes. They fool our body into believing that it is under attack by a new pathogen. A typical all-out defence response ensues, arming the body with required T & B-cells and antibodies that thereafter remain at its rapid disposal for a very long time.

So how is this intelligence fed? It’s via the vaccine core which is either injected or given orally to an individual. It contains the pathogen itself in one shape or form. However, it is possible that the micro-organism is weakened to such an extent that it loses its ability to cause disease but maintains its skills to enter the body— as in the case of the MMR vaccine. It is also possible that it is treated with heat or chemicals and killed— such as in the case of the hepatitis-A vaccine. Another method is that, small selective parts of the virus are stripped and used— this method is employed in one of the flu vaccines. Lastly, another way of feeding the vaccine core to the body is that a small part of the virus is artificially prepared and used. This method is used for the hepatitis-B vaccine.

At the back of Covid-19 pandemic, another type of vaccine has become a household name. Messenger RNA (mRNA) vaccine is an intelligent development where instead of the virus or a part of it, a genetic message (called mRNA) fills the vaccine core. When given as an injection, mRNA gets absorbed in the body and is taken up by some of the human cells. These cells get fooled into believing that this mRNA is an instruction from its own control centre— the nucleus. It is passed over to the human cellular machinery, leading to the manufacturing of a protein that is a replica of one already present on the virus’s outer layer. The protein (called S for spike) is then taken up by the adaptive immunity which mounts a full response as described above. Viral mRNA can also be delivered to human cells by inserting it first into a non-disease-causing virus which acts as a courier and transports it to the cells. These vaccines are called vector vaccines.

Different types of Covid-19 vaccines are currently in development. They range from mRNA vaccine to killed-whole-virus vaccines. Whole virus vaccines (whether killed or live) have the advantage of priming the immune system against “all” viral proteins whilst mRNA only creates one. Responses are likely to be better in the former group but it takes months for these vaccines to be developed, let alone test their safety and efficacy. mRNA vaccines on the other hand, can be created in a matter of weeks. If a change in virus structure forces a vaccine redesign, a new mRNA vaccine takes days, others a much longer time, likely to be in months. Reported side effects from Covid-19 mRNA virus vaccines are no more than those that have been traditionally associated with any viral vaccine.

The writer is a Consultant Virologist at Sheffield, KEMCA UK