Vaccination

Vaccination is the process of giving a person a preparation, called a vaccine, that trains the immune system to recognise and resist a specific infectious disease. The goal is to create protection, or immunity, without the person having to suffer the illness itself.

Vaccination is widely regarded as one of the most effective public health measures ever developed. It has helped eliminate or sharply reduce diseases that once killed or disabled millions of people each year.

How vaccines work

The immune system learns to fight germs by encountering them. When a real infection occurs, the body produces antibodies and specialised cells that target the invader, and it keeps a memory of it so that a later encounter can be fought off quickly. A vaccine mimics this learning process using a harmless form of the germ or a piece of it.

Because the vaccine does not cause the disease, the person gains the benefits of immune memory without the danger. Some vaccines require several doses or periodic boosters to build or maintain strong protection.

History

Before modern vaccines, a practice called variolation exposed people to a mild form of smallpox to build resistance. In 1796 the English physician Edward Jenner showed that inoculation with material from cowpox could protect against smallpox, a safer approach that gave rise to the word vaccine, from the Latin for cow.

In the nineteenth century Louis Pasteur and others developed vaccines against diseases such as rabies, building on the growing understanding of germs. The twentieth century brought many more vaccines, including those against diphtheria, tetanus, polio, measles and others. Smallpox was declared eradicated in 1980 after a global campaign, the first human disease to be wiped out by vaccination, and polio has been pushed close to elimination.

Types of vaccines

Vaccines come in several forms. Live attenuated vaccines use weakened germs that cannot cause serious illness but provoke a strong immune response; measles and some other vaccines are examples. Inactivated vaccines use killed germs. Subunit and toxoid vaccines use only specific proteins or inactivated toxins. Some newer vaccines use genetic instructions, such as messenger RNA, that tell the body's own cells to produce a harmless piece of a germ for the immune system to react to.

Each approach has trade-offs in how strongly and how long it protects, and in how it is stored and delivered.

Herd immunity and impact

When enough people in a community are immune, a germ finds it hard to spread, which protects those who cannot be vaccinated, such as infants or people with certain medical conditions. This indirect protection is called herd immunity. The proportion needed varies by how contagious a disease is; measles, being highly contagious, requires a very high level of coverage.

Vaccines have prevented countless cases of disease, disability and death. They have also freed resources, as preventing illness is far cheaper than treating it.

Safety and common concerns

Vaccines are tested in clinical trials and monitored continuously after approval for rare side effects. Most reactions are mild and short-lived, such as soreness at the injection site or a low fever. Serious reactions are very rare, and the risks of the diseases themselves are generally far greater than the risks of vaccination.

Some people worry, for example, about a supposed link between vaccines and autism, an idea based on a study that was later shown to be fraudulent and retracted, and which has been contradicted by many large studies. Public health bodies continue to stress that the benefits of vaccination outweigh the risks. Like all medicines, vaccines are not risk-free for every individual, and people with particular health conditions should seek advice.

Looking ahead

Research continues into vaccines against difficult targets such as HIV, malaria and various cancers. Rapid vaccine development during recent pandemics showed how quickly science can respond when needed, while also highlighting the importance of trust, access and fair distribution.

Development and global use

Bringing a vaccine to the public follows a long path. Researchers first explore whether a candidate is safe and triggers an immune response, then test it in larger trials to measure how well it protects against disease. Regulators review the results before approval, and monitoring continues afterwards.

Distributing vaccines widely is a logistical challenge. Many must be kept cold from factory to clinic, a requirement known as the cold chain. National immunisation schedules set out which vaccines children and adults should receive and when. International bodies and partnerships work to fund and deliver vaccines in poorer countries, where the burden of preventable disease is often greatest.

Coverage is uneven, and outbreaks of diseases such as measles have returned in places where vaccination rates have fallen. Vaccine confidence is therefore treated as an essential part of public health, alongside supply and access.

Key points