Key points
- Vaccination protects against harmful diseases before contact, by training the immune system (WHO).
- Vaccines are tested in stages — laboratory and animal studies, then phased trials in people, then regulatory review and monitoring after introduction.
- Global coverage in 2025: 90% of children received a first dose of DTP vaccine and 85% received all three doses; 13.5 million children received no dose (WUENIC, July 2026).
- This page gives official information only. It does not build a personal schedule and does not advise what any individual should take or when.
Direct answer: Vaccination protects against harmful diseases before contact, by training the immune system to recognise a germ without causing the disease. This page gives official information only: how vaccines are tested, what global coverage looks like, and where to find your country’s official schedule. It does not build a personal schedule and does not advise what any individual should take or when.
What vaccination is
A vaccine introduces a harmless form of a germ — or a part of it, or instructions for making a part of it — so the immune system learns to recognise it. If the real germ arrives later, the immune system responds faster and more strongly than it would have without that training. That is the whole mechanism, in one paragraph, as WHO describes it.
The protection is not only for the person vaccinated. When enough people in a community are protected, a disease finds fewer people to spread through, and people who cannot be vaccinated — newborns, people with certain medical conditions — are less likely to meet the germ at all. That population effect is why coverage is reported for whole countries, not just for individuals.
How vaccines are tested
Vaccines are tested in stages, and each stage must be passed before the next begins. First come laboratory studies and animal studies, which check that the vaccine produces the intended immune response and shows no unexpected harm. Then come clinical trials in people, in phases: small groups first, to check safety and the immune response; then larger groups, to check safety in more people and to measure how well the vaccine prevents disease; then very large trials, to confirm the effect and catch rarer side effects.
Only then does a regulator review the evidence — the trial data, the manufacturing process, and the quality controls — and decide whether the vaccine can be used. And testing does not stop at approval: safety is monitored after introduction, through reporting systems that collect reports of side effects from health workers and the public, and through studies that compare vaccinated and unvaccinated groups over time. A vaccine that shows a problem after introduction can have its use restricted or stopped.
That sequence is the same in outline everywhere, though the regulators differ by country: the European Medicines Agency in the EU, the Food and Drug Administration in the United States, the Medicines and Healthcare products Regulatory Agency in the United Kingdom, and WHO’s own prequalification programme, which assesses vaccines for use in lower-income countries.
Global coverage in 2025
The WHO/UNICEF estimates of national immunisation coverage (WUENIC), published in July 2026, put global coverage in 2025 at 90% of children receiving a first dose of diphtheria-tetanus-pertussis vaccine (DTP1) and 85% receiving all three doses (DTP3). The gap between those two numbers is children who start the series and do not finish it. And 13.5 million children received no dose at all — the “zero-dose” children, used as the marker for children entirely missed by routine immunisation.
Coverage is published annually, every July, and this page will be re-checked when the next edition lands. A figure older than its re-check interval is labelled stale and re-checked before the page is re-promoted.
Finding your country’s schedule
Schedules differ by country: the same vaccine is given at different ages in different places, and some vaccines are on one country’s schedule and not another’s. The Vaccination Directory on this site links to the official source for each country — WHO and UNICEF for global data, the CDC for the United States, the NHS for the United Kingdom, and the national authorities for Canada, Australia and New Zealand — so a reader can check the schedule that applies where they live. Use the official source, not a summary: schedules change, and only the official page is current.
A note on the CDC schedules
The CDC’s immunisation schedules changed during 2025 in the United States, and those changes are contested and litigated. They are US-only: they do not apply outside the United States, and they must never be presented as worldwide advice. Any CDC mention on this site is date-stamped and framed as the US schedule, not as a general recommendation.
What this page does not do
This page does not build a personal schedule, does not say which vaccines any individual should take or when, and does not answer questions about individual concerns. Those are questions for a qualified health professional and for the official schedule of the country concerned. The page’s job is narrower: to explain, from official sources, how vaccines are tested and how coverage is measured.
Frequently asked questions
Does this page tell me which vaccines I need?
No. It gives official information only and does not build a personal schedule. Your country's official schedule is linked from the Vaccination Directory on this site.
How are vaccines tested?
In stages: laboratory and animal studies, then phased clinical trials in people, then review by a regulator, then monitoring after introduction. Each stage must be passed before the next begins.
What does zero-dose mean?
A child who received no dose of DTP vaccine — used as the marker for children entirely missed by routine immunisation. In 2025 there were 13.5 million zero-dose children worldwide.