Vaccines: Teaching the Immune Response in Advance

Vaccination prepares adaptive immune responses without requiring the full risks of the target disease. Antibodies, B cells and T cells may contribute. Protection takes time, may need several doses and differs between people.

Vaccination and immunisation are related but different Vaccination is the administration of a vaccine. Immunisation is the process through which a person develops protection after vaccination or another immune exposure. The distinction matters because receiving a vaccine does not guarantee the same response in everyone. Age, health, medicines, timing and the vaccine itself can affect protection. Vaccines reduce the risk of specific diseases. They do not strengthen every part of the immune system or protect equally against unrelated infections. What a vaccine presents to the immune system An antigen is a substance recognised by the immune system. Vaccines present selected antigens or give cells temporary instructions for making an antigen. Some vaccines contain an inactivated organism. Others contain selected proteins, sugars, inactivated toxins or genetic instructions delivered by mRNA or a viral vector. A live attenuated vaccine contains a weakened living organism that can replicate to a limited extent. Suitability depends on the vaccine and the person's health. Some live vaccines are unsuitable during significant immunosuppression or pregnancy. Inactivated, subunit and mRNA vaccines cannot reproduce as the target organism. These platforms use different routes to produce a useful immune response. No single description, such as a harmless fragment, accurately describes every vaccine. Antibodies are important, but they are not the whole response Vaccination first activates innate immune signals. Antigen presenting cells then help activate the adaptive immune response. A B cell can develop into an antibody producing cell. An antibody binds a particular antigen and can block infection or help remove the infectious agent. A T cell can support B cell responses or recognise infected cells. The relative importance of antibodies and T cells differs between vaccines and diseases. Some activated B cells and T cells become long lived memory cells. This immune memory can support a faster or stronger response after later exposure. Immune memory is not identical to permanent protection. Antibody levels, memory cell responses and protection can change over time. Vaccine protection has several possible outcomes A vaccine may reduce the chance of infection, symptomatic disease, severe illness, complications or onward transmission. These outcomes are related but not interchangeable. Some vaccines are highly effective at preventing infection. Others protect more strongly against severe disease than against acquiring or transmitting the organism. A vaccinated person can therefore develop a breakthrough infection. This does not automatically show that the vaccine provided no benefit. It is usually impossible to know how severe one person's illness would have been without vaccination. Vaccine effectiveness is estimated by comparing outcomes across suitable groups. No vaccine provides complete protection to every recipient. A recent vaccination also does not exclude another diagnosis when symptoms develop. Protection takes time and may need several doses A primary series is the first group of doses needed to build the intended response. Some vaccines need one dose, while others need several spaced doses. Protection is not immediate. The time needed varies, and completing the recommended schedule provides the best expected protection. A booster dose is an additional dose given after the primary series. It may restore waning protection or strengthen a response that needs further stimulation. A booster may also update protection when important strains change. It does not prove that every earlier dose failed. However, some people do not develop the expected response after an initial course. Additional doses or another strategy may be advised for particular groups. Population level protection is conditional Population level protection occurs when immune people reduce opportunities for a transmissible infection to spread. This can indirectly lower exposure for other people. The effect depends on vaccine uptake, vaccine effectiveness against infection or transmission, duration of protection and how people mix. The organism's contagiousness also matters. There is no single coverage threshold that applies to every infection. Falling uptake or waning protection can allow outbreaks to return. Not every vaccine creates population level protection in the same way. Tetanus does not spread from person to person, so tetanus vaccination mainly protects the vaccinated individual. Indirect protection is valuable but never guarantees safety for someone who cannot be vaccinated or does not respond adequately. Side effects and adverse events need careful interpretation Most vaccine side effects are mild and short lived. Common examples include soreness, tiredness, headache or a raised temperature. Serious adverse reactions are possible but rare. Vaccine recommendations balance these risks against the harms prevented by vaccination. An adverse event after vaccination is any health problem occurring afterwards. Timing alone does not prove that the vaccine caused it. Having side effects does not prove stronger protection. Having no side effects does not show that the vaccine failed. Tell the vaccination team about a previous severe allergic reaction, significant immunosuppression, pregnancy or another concern that may affect vaccine choice. Why the flu vaccine is offered repeatedly Seasonal influenza viruses change over time, and protection also declines. The vaccine formulation and programme are therefore reviewed for each season. The injected flu vaccines used in the UK are non live and cannot cause flu. The nasal vaccine is live attenuated and has different suitability rules. The flu vaccine can reduce the chance of flu and serious illness, but infection can still occur. Protection usually takes up to fourteen days to develop. Mild aching or fever after vaccination is an expected side effect, not influenza caused by an injected vaccine. A coincidental respiratory infection can also occur. Taking part in vaccination decisions Check which vaccines and doses are currently recommended for your age, health, pregnancy status, work, travel or other risk. UK schedules change when evidence changes. Ask what outcome the vaccine mainly prevents and when protection is expected. Ask whether further doses or later boosters are recommended. Discuss previous reactions and medicines that affect immunity. Do not assume that every live or non live vaccine has the same restrictions. Call 999 for sudden swelling of the lips, mouth, tongue or throat, severe breathing difficulty, collapse or another possible anaphylactic reaction. This lesson explains general vaccine biology. It cannot determine which vaccine, formulation or schedule is appropriate for you.

Vaccines prepare antigen specific immune responses before later exposure. Their effects vary across infection, disease, severity and transmission. Protection takes time, may require several doses and remains incomplete for some people.