Health Original explainer
A vaccine is a training session for your immune system. How it works, and the four phases of testing behind it.
When a vaccine is injected into your arm, nothing dramatic happens. There is a small prick, a little soreness, and life goes on. Inside your body, though, a training session has just begun. The vaccine shows your immune system a harmless preview of a germ — a killed virus, a weakened one, or just a fragment of it — and your immune system learns to recognise it. If the real germ ever arrives, your body already knows what to do. That is the whole idea of vaccination.
Your immune system is a defence force with a memory. The first time it meets a new germ, it is slow: it has to work out what the invader looks like and how to fight it, and while it learns, you fall ill. The second time, it is fast. It remembers the germ and attacks before the infection can take hold. A vaccine gives your body that memory without the illness. It introduces a safe version of the germ, or a piece of it, called an antigen. Your immune cells study the antigen, build defences against it, and remember it. The memory can last for years, sometimes for life.
Not all vaccines work the same way. The oldest kind uses a killed or inactivated germ: the virus or bacterium is dead, so it cannot cause disease, but its shape still teaches the immune system. Covaxin, one of India's Covid vaccines, was made this way. Another kind uses a live but weakened germ, which gives a strong immune response but is not suitable for everyone. A third kind uses a harmless virus as a delivery vehicle — a viral vector — to carry the instructions for making one piece of the germ. Covishield, the other Covid vaccine used in India, was made this way. The newest kind, messenger RNA or mRNA vaccines, delivers instructions that tell your own cells to make a single harmless piece of the germ, and your immune system learns from that.
Different designs suit different diseases. Killed vaccines are safe and well understood but sometimes need booster doses. Live weakened vaccines give long-lasting protection but cannot be given to people with weak immune systems. mRNA vaccines can be designed quickly once scientists know the genetic code of the germ, which is why they were ready so fast during Covid. None of these is a perfect technology. All of them share the same principle: show the immune system a safe preview, and let memory do the rest.
Before any vaccine reaches the public, it goes through years of testing. It starts in the laboratory and in animals, where scientists check that it triggers an immune response and does not cause obvious harm. Only then do human trials begin, in three phases. Phase 1 involves a small group — typically 20 to 100 healthy volunteers — and asks the most basic questions: is it safe, and does it trigger an immune response? Researchers try different doses and watch closely for side effects.
Phase 2 expands to several hundred volunteers, including people from the groups who will actually receive the vaccine — different ages, and people with common health conditions. This phase pins down the right dose and the right gap between doses, and keeps checking safety. Phase 3 is the big one: thousands of volunteers, sometimes tens of thousands, split randomly into two groups. One group gets the vaccine, the other gets a placebo — a harmless dummy injection. Neither the volunteers nor the researchers know who got which, a method called double-blinding. The trial then waits to see who falls ill. If far fewer people fall ill in the vaccinated group, the vaccine works. Only after all this does a regulator approve it. Testing continues even after approval, in Phase 4, watching for rare side effects in the wider population.
India ran the largest vaccination drive in its history against Covid-19, and it is worth understanding what happened. The drive began on 16 January 2021, using two vaccines made in India: Covishield and Covaxin. A digital platform called CoWIN handled registration, appointments, and certificates, so that a billion-plus people could be scheduled and tracked. By April 2023, India had administered about 220.66 crore doses — more than 2.2 billion. About 97 per cent of eligible people had received at least one dose, and about 90 per cent had received two.
The speed of the Covid vaccines surprised many people, and it fed a rumour that safety had been skipped. It had not. The phases were not skipped — they were run in parallel, with money and urgency that normal trials never get. Manufacturing was scaled up before trials finished, at the companies' own risk, so that doses were ready the day approval came. The testing was as rigorous as ever. What changed was the waiting, not the science.
Vaccines are powerful but not perfect. No vaccine protects everyone, and protection can fade over time, which is why boosters exist. A vaccinated person can still catch an infection, though usually a milder one. Vaccines also do not work if too few people take them: when coverage is low, the germ keeps circulating and finds the unprotected. That is why public health officials talk about community protection — when enough people are immune, the germ struggles to spread, and even those who cannot be vaccinated are shielded. Side effects exist, mostly a sore arm or a day of fever, and very rarely something more serious. Regulators track these constantly, and the record for approved vaccines is one of the safest in all of medicine. The lesson is simple: a vaccine is not a magic shield. It is a training session for your immune system, tested in stages, and refined over decades. And it only works if people take it.
This explainer was written by The Daily Short's desk from the sources listed above. It is general information, not professional advice. Spot an error? Tell us via the Contact page and we will correct it. Follow our official Instagram @the.daily.short.