Many vaccines need more than one dose to provide protection. This blog post explains why multiple doses are sometimes needed, how vaccine schedules are determined, and what the evidence says about staying protected.
Why is one dose of a vaccine sometimes not enough?
When you get a vaccine, your immune system develops cells that are programmed to remember the targeted infection. If you later come into contact with the infection your cells will recognise it and fight it off. It can take from a few days up to 2 or 3 weeks for the body to respond, and sometimes one dose will not lead to strong or lasting protection. Further doses can then build on the initial response. The number of doses needed depends on the type of vaccine and the immune response it brings about.
How does the type of vaccine affect how many doses are needed?
There are 2 main kinds of vaccine, each with different dose requirements.
- Live-attenuated vaccines contain very weakened but living bacteria or viruses. These vaccines cause the body to protect itself in a way that is very like the way the body responds to natural infections. This means live vaccines tend to produce strong and long-lasting protection – in some cases this is lifelong with fewer doses. However, these vaccines are not suitable for some people with very weakened immune systems.
- Non-live vaccines use killed pathogens, parts of these bugs that the body responds to, or treated bacterial toxins. These vaccines usually need more than one dose to give strong and lasting protection. They are suitable for people with weakened immune systems. The tetanus vaccine is an example, requiring doses throughout infancy and childhood, with booster doses continuing into adulthood.
Why must doses be spaced apart rather than given together?
The spacing of doses allows the body to complete its response to the vaccine before any follow-up is given, so the strengthening effect of subsequent doses is fully achieved. Vaccine schedules set out how many doses are needed and when to have them. Based on the way the vaccine works, the way the body responds and when the risk of disease is greatest. It is important to follow the recommended schedules as closely as possible and to catch up on any missed doses as soon as you can.
What is the difference between a primary course and a booster?
A primary course is the initial series of doses required to establish immunity. Booster doses are administered at a later stage to reinforce protection and sustain it for the long term.
Why do some vaccines, such as the seasonal flu vaccine, need to be taken annually?
Some viruses change over time, meaning that protection given by an earlier vaccine may not work as well against a newer strain of virus. The seasonal flu vaccine is made each year to target the most common strains in the coming season. COVID-19 vaccines are also updated regularly based on circulating variants. It is important for those eligible to have the vaccine every year (or twice per year for COVID-19), even if they have had one before.
Is it still possible to catch a disease after being vaccinated?
Immunity takes several weeks to develop following vaccination, during which time an individual may still be vulnerable to infection. Most people will make a good, long-lasting response to vaccines but there may be a small number of people who do not respond as well or where immunity wanes over time. People are however usually much less likely to get seriously unwell from infections they have had vaccines against.
What happens if someone misses one or more doses of a vaccine?
Everyone should receive vaccines at the recommended times whenever possible. Vaccines are carefully scheduled for children and adults to match when their body is able to mount the best immune response to them and the period when they are most at risk from a particular infection. However, it is usually possible to catch up.
Children and adolescents who have missed doses should receive catch-up vaccinations as soon as possible. Children and adults will become eligible for some vaccinations as they reach certain ages, and may also need vaccines because of health conditions they develop, for their occupation or for travel. As a baseline, all adults are advised to follow updated seasonal flu and COVID-19 vaccination guidance.
For advice on which vaccines are recommended for you or your family, consult your GP or refer to the NHS vaccination schedule.