Active vs Passive Immunity and Vaccines
Active immunity is protection produced by the body’s own immune system after exposure to an antigen, through infection or vaccination, while passive immunity is temporary protection obtained by receiving ready-made antibodies from another source. In brief: active immunity takes days to weeks to develop but can last for years because memory cells are formed. Passive immunity works immediately but fades in weeks to months because no memory cells are made. A baby receives passive immunity from its mother through the placenta and breast milk.
Vaccines have saved more lives than any other medical intervention except clean water. This guide explains the four types of immunity, how vaccines work and the types of vaccine, what herd immunity is and how safe vaccines are tested, compares active and passive immunity, works through a vaccination coverage example and corrects common mistakes.
What is active immunity?
In active immunity, the person’s own lymphocytes respond to an antigen, and memory cells are produced. It develops in two ways.
- Natural active immunity: develops after an infection. A person who has had chickenpox is generally immune to it in the future.
- Artificial active immunity: develops after vaccination, without suffering the disease.
It takes time, because the primary response takes days to weeks, but protection can last for many years or a lifetime, depending on the disease and the vaccine. Booster doses extend protection for some vaccines, such as tetanus.
What is passive immunity?
In passive immunity, antibodies are supplied from outside, and the person’s own immune system is not involved. There are no memory cells, so protection is short-lived: the antibodies are broken down within weeks or months.
- Natural passive immunity: antibodies (mainly IgG) pass from the mother to the fetus across the placenta, and IgA antibodies are passed in breast milk, especially in colostrum, the first milk. They protect the baby during the first months of life, while its own immune system is developing.
- Artificial passive immunity: injection of antibodies made in another person or animal. Examples are antivenom for snake bites, rabies immunoglobulin after exposure, and antitoxin for tetanus or botulism. Monoclonal antibodies are used in some treatments.
Passive immunity is useful when immediate protection is needed, or when the person cannot make antibodies.
Active and passive immunity compared
| Feature | Active immunity | Passive immunity |
|---|---|---|
| Source of antibodies | Own plasma cells | Another person or animal |
| Memory cells | Formed | Not formed |
| Speed of protection | Slow (days to weeks) | Immediate |
| Duration | Long (years to lifetime) | Short (weeks to months) |
| Natural example | Recovering from measles | Antibodies from mother |
| Artificial example | Vaccination | Antivenom injection |
How do vaccines work?
A vaccine exposes the immune system to an antigen from a pathogen in a safe form. The body responds with a primary response, producing antibodies and memory B and T cells. If the person later encounters the real pathogen, the secondary response is fast and strong, usually preventing disease. This is the same mechanism as in natural infection, but without the risk of illness.
Some vaccines contain an adjuvant, a substance (such as an aluminum salt) that improves the immune response. Several doses may be needed to provide full protection, because the first dose generates the primary response and the following dose the stronger secondary response.
What types of vaccine exist?
- Live attenuated vaccines: contain a weakened form of the live pathogen. They give strong, long-lasting immunity but are not suitable for people with weakened immune systems. Examples: measles, mumps and rubella (MMR), yellow fever.
- Inactivated (killed) vaccines: contain a pathogen that has been killed. They are safer but may need boosters. Examples: polio injection, hepatitis A, rabies.
- Subunit, recombinant and conjugate vaccines: contain only part of the pathogen, such as a protein or a sugar. Examples: hepatitis B, HPV, whooping cough (acellular).
- Toxoid vaccines: contain an inactivated toxin. Examples: tetanus, diphtheria.
- mRNA and viral vector vaccines: deliver genetic instructions for cells to make a harmless antigen, such as the spike protein of SARS-CoV-2. mRNA vaccines do not enter the cell nucleus and do not change a person’s DNA, and the mRNA is broken down within days.
What is herd immunity?
When a large enough proportion of a population is immune, the pathogen finds it hard to spread, and this also protects people who cannot be vaccinated, such as newborns and people with weak immune systems. The threshold depends on how contagious the disease is, as explained in the article on epidemiology. For measles, about 95 percent coverage is needed, and for polio about 80 to 85 percent.
Worked example: how many people are protected?
A town of 20 000 people has a measles vaccination coverage of 90 percent. The vaccine is 97 percent effective after two doses. The herd immunity threshold for measles is about 94 percent, assuming R₀ = 16.
- Vaccinated: 20 000 × 0.90 = 18 000 people.
- Protected by the vaccine: 18 000 × 0.97 = 17 460 people.
- Immune share of the population: 17 460 ÷ 20 000 = 0.873, or 87.3 percent (ignoring people with natural immunity).
- Compare with the threshold: 87.3 percent is lower than 94 percent. Therefore the town is at risk of outbreaks.
- Coverage needed: to reach 94 percent immune with a 97 percent effective vaccine, coverage must be 0.94 ÷ 0.97 = 0.969, or about 97 percent.
The unprotected population is 20 000 − 17 460 = 2 540 people, who include those who are unvaccinated, and those whose vaccination did not work. This is why high coverage matters even for effective vaccines. The numbers are illustrative, and real populations are not evenly mixed, so a community with low coverage can have an outbreak even if the national figure is high.
How is vaccine safety ensured?
Vaccines undergo laboratory tests, then clinical trials in three phases with increasing numbers of volunteers, often tens of thousands in the last phase, before approval. Regulators review the data, and safety is monitored continuously after approval. Common side effects, such as a sore arm, mild fever and tiredness, are signs that the immune system is responding and usually pass within a few days. Serious allergic reactions are rare, at roughly one case per million doses for many vaccines. Vaccines are tested in large studies, and the evidence has not found a link between vaccines and autism: the 1998 paper that claimed it was retracted and found to be fraudulent.
How were vaccines developed?
Edward Jenner showed in 1796 that exposure to cowpox protected against smallpox, and he called the procedure vaccination, from the Latin vacca, meaning cow. Louis Pasteur developed vaccines for chicken cholera, anthrax and rabies in the 1880s. Jonas Salk’s polio vaccine was licensed in 1955, and Albert Sabin’s oral vaccine followed. The World Health Organization declared smallpox eradicated in 1980. The first mRNA vaccines were authorized for COVID-19 in late 2020.
Which mistakes are common?
Mistake 1: Saying that vaccines give the person ready-made antibodies
Vaccines stimulate the person’s own immune system to make antibodies and memory cells, so they provide active immunity.
Mistake 2: Thinking that antibodies from the mother last for life
They fade within months. The baby needs vaccination to develop its own immunity.
Mistake 3: Believing that vaccination with a killed vaccine can cause the disease
Inactivated vaccines cannot cause the disease they protect against.
Mistake 4: Confusing natural and artificial
Natural means by infection or from the mother. Artificial means by medical intervention (vaccine or injected antibodies).