Vaccines and immunisation: how they work, how safe they are

Vaccination is a core part of modern medicine, often called allopathy, and aims to prevent illness rather than treat it. This article explains how vaccines work, how their safety is tested and watched, what side effects to expect and what India's national programme offers. It uses information from the World Health Organization (WHO), the UK National Health Service (NHS), UNICEF India and the US Centers for Disease Control and Prevention (CDC).

1. Introduction

WHO describes vaccination as a way to protect people from harmful diseases before they meet them. It works with the body's natural defences to build resistance to specific infections ^[1]. Vaccines are recommended across life, from birth through childhood, adolescence and older age ^[1]. WHO also says that many countries issue a vaccination card that lists the vaccines received and the doses still due ^[1].

2. How vaccines work

A vaccine trains the immune system, the body's defence network. WHO explains that vaccines contain killed or weakened germs, parts of germs, or genetic instructions that tell cells to make a harmless piece of the germ, so they do not cause the disease. The immune system recognises the germ, makes antibodies (proteins that attack it) and remembers how to fight it. If you meet the germ later, the body can destroy it before illness develops ^[1]. Protection can last years, decades or a lifetime after one or more doses ^[1].

The NHS describes two broad kinds. Live vaccines use weakened organisms, often give long-lasting protection and cannot be given to people with weakened immune systems. Non-live vaccines use destroyed organisms, can be given to people with weakened immune systems, but may need several doses or boosters ^[3].

Most vaccines are injected, and some are taken by mouth or sprayed into the nose ^[1]. Combination vaccines put several vaccines in one shot, and WHO says they are as safe and effective as separate ones ^[1].

3. Protecting others

Some people cannot be vaccinated, including very young babies, people who are seriously ill and people with certain allergies. They depend on those around them being vaccinated ^[1]. The NHS calls this herd protection: when enough people are vaccinated, a disease struggles to spread ^[3]. The NHS notes that diseases can return quickly if vaccination rates fall ^[3].

4. How safety is tested and monitored

WHO describes testing in stages ^[2]:

• Laboratory testing: the vaccine and its ingredients are examined for safety and effectiveness.
• Phase 1: about 20 to 50 people, to study safety, side effects and how it is given.
• Phase 2: several hundred people who resemble the intended recipients.
• Phase 3: thousands of people, to confirm safety and effectiveness.

National regulators oversee the trials and decide on licensing. Each country's regulator also decides whether a vaccine joins its national programme ^[2]. After approval, vaccines are monitored at local, national, regional and global levels ^[2]. The NHS notes that the UK regulator keeps checking for rare side effects, and anyone can report a suspected side effect ^[3].

An adverse event following immunisation is any unexpected medical problem after a vaccine, which does not necessarily mean the vaccine caused it. WHO says an independent group of experts assesses serious events. Health events after vaccination most often turn out to be coincidental, and some are traced to storage, transport or administration errors ^[2].

5. Side effects and who should check first

The NHS says most side effects are mild and short-lived. They include soreness, redness or swelling where the shot was given for 2 to 3 days, feeling unwell or having a high temperature for 1 to 2 days, tiredness and headache ^[3]. Serious allergic reactions are rare and usually begin within minutes, and vaccinators are trained to treat them ^[3].

Vaccines should generally not be given to people who had a serious allergic reaction to an earlier dose or an ingredient, and some people with weakened immune systems cannot have certain vaccines ^[3]. WHO adds that people with a severe illness and high fever on the day of vaccination should check with a health worker, and that people who have missed doses should ask a health worker about catching up ^[1]. The sources also answer common worries. WHO says many large, high-quality studies have found that vaccines do not cause autism ^[2]. WHO says giving several vaccines at one visit does not overwhelm the immune system ^[1], and the NHS also considers it safe ^[3].

6. India's Universal Immunization Programme

UNICEF India lists the vaccines given in the national programme, which protect against tuberculosis, polio, hepatitis B, diphtheria, whooping cough, tetanus, Haemophilus influenzae type b, rotavirus diarrhoea, pneumococcal disease, measles, rubella and Japanese encephalitis ^[4]. The schedule includes:

• At birth: BCG, oral polio and hepatitis B birth dose.
• At 6, 10 and 14 weeks: a series including the pentavalent, polio, rotavirus and pneumococcal vaccines.
• At 9 to 12 months: measles and rubella, with Japanese encephalitis only in districts with a high disease burden.
• At 16 to 24 months and 5 to 6 years: booster doses.
• At 10 and 16 years: tetanus and diphtheria boosters.
• In pregnancy: tetanus and diphtheria doses, given as early as possible in pregnancy and at least 4 weeks apart ^[4].

Schedules can change, so confirm the current one with your doctor or health centre. Adults also need updates, because childhood protection can fade and adult risks differ. The CDC advises yearly flu vaccination for everyone aged 6 months and over, especially people at higher risk ^[5].

7. When to see a doctor

Speak to a doctor, nurse or health worker if you have missed doses, are pregnant, have a weakened immune system or a past severe allergy, or are unsure what you need ^{[1, 3]}. Seek advice if a reaction seems more than mild or lasts longer than expected, and report it to the health worker who gave the vaccine ^{[2, 3]}. In an emergency, call your local emergency number (112 in India, 911 in the United States).

8. Conclusion

Vaccines train the body to defend itself before illness begins. They are tested in several phases and monitored for safety for as long as they are in use. Most side effects are mild. Keeping your own and your children's vaccination records up to date, and discussing any concern with a health worker, is the most practical step.