Vaccination is a very effective measure for providing immunity to many infectious diseases. The discovery of vaccines played a central part in the eradication of smallpox and helped significantly reduce the incidence of potentially severe diseases such as poliomyelitis and measles. Live vaccines (attenuated, i.e. noninfective pathogens) or inactivated vaccines (subunits or complete pathogens) are used to achieve active immunization, which enables the host's immune system to build up a sustained immune response to specific pathogens. The immune response may be measured and quantified by assessing the antibody titer. In the event of potential disease (e.g., after exposure to high-risk pathogens), if the immune system is unable to produce sufficient antibodies fast enough, passive immunization can offer immediate short-term protection via direct injection of pooled antibodies for many conditions. Modern vaccines are usually well-tolerated, and adverse events are rare. However, the intervals between vaccine administration and possible contraindications must be considered.
- A product (e.g., dead or weakened organism) that provides immunity from a disease
- May be administered through injection, orally, or nasally
- Vaccination: administration of a vaccine that induces an active immune reaction in form of cellular and/or humoral response, providing immunity against a pathogen
- The process by which a person becomes protected from a disease
- Vaccines and recovering from some infections cause immunization.
Aims of routine immunization
- Herd immunity: Once a certain percentage of the population has received immunization, non-vaccinated individuals (e.g., children too young to receive vaccination) will also be protected.
- Eradication of disease
- Lower incidence and associated risks: The Haemophilus influenzae type b (Hib) vaccine has decreased the number of cases of invasive Hib disease (e.g., pneumonia, bacteremia, meningitis, epiglottitis, infectious arthritis) in children younger than 5 by more than 99%.
- Mechanism of action
- Indications 
- Application: Vaccines are available for intramuscular as well as for intravenous injection .
- Simultaneous vaccination: ≥ 1 vaccine administered on the same day, but in different syringes and at different anatomical locations 
- After administration of a passive vaccine against a specific pathogen, a live vaccine against the same pathogen should not be administered for at least 3 months. 
- In active immunity, the body's immune system reacts to the presence of antigens by producing antibodies.
- In general, a combination of different active vaccinations is possible.
- Slow onset, but immunity usually lasts for years or even a lifetime.
- Besides vaccines and toxoids, natural infections lead to active immunization as well.
Current vaccination recommendations for the US can be found in the .
- Definition: Modified functioning virus or bacterium that can replicate in the patient's body but does not cause disease.
- Mechanism of action
- Oral vaccine or subcutaneous/intramuscular injection in children > 12 months
- Not indicated in children < 9 months; (the rotavirus vaccine is an exception, which is first given at 6 weeks of age)
- Second dose usually recommended to “catch” non-responders (not as a boost)
- Multiple live vaccines can be given simultaneously, but if given at different times they should be at least 4 weeks apart to avoid possible interference.
- May be administered simultaneously with inactivated vaccines
- MMR: prevents measles, mumps and rubella infections
- Varicella: against varicella-zoster virus (VZV)
- Zoster: prevents reactivation of latent VZV (i.e., shingles)
- Yellow fever
- Influenza (intranasal)
- Oral polio, Sabin (no longer available in the US)
- Typhoid; (oral, Ty21a)
- Very rarely, live attenuated vaccines may become virulent again; and are thus often contraindicated in immunodeficient individuals and pregnant women .
- HIV-positive individuals can be vaccinated with live attenuated vaccines (e.g., MMR and varicella) if their CD4 cell count is ≥ 200 cells/mm3.
- Adenovirus vaccine is given in its nonattenuated form to military recruits.
|Whole vaccines||Fractional vaccines|
|Mechanism of action|| |
|Special considerations|| |
Common adverse effects 
- Affects ∼ 1/3
- Usually begin within the first 48–72 hours after administration and last 1–2 days
- Live attenuated vaccine: can cause mild form of the disease, usually appearing within 1–3 weeks of administration; : usually caused by replication of the attenuated vaccine strain
Rare adverse effects
- Serious allergic reaction (generally < 1 per million doses)
- Live attenuated vaccine: attenuated course of the disease following immunization (e.g., vaccine-related measles)
- Vaccine injury (∼ 1/1,000,000): permanent injury from a vaccination or a vaccine-related complication (e.g., encephalopathy, seizures, brachial neuritis)
There is no link between autism and vaccines or their ingredients. 
- General contraindications 
Live vaccinations are not recommended for
- Pregnant individuals
- Immunodeficient individuals
Special contraindications 
- Rotavirus vaccine: severe combined immunodeficiency (SCID), history of intussusception; , uncorrected GI tract malformation (e.g., Meckel's diverticulum) 
- Tetanus: development of Guillain-Barré syndrome within 6 weeks after a previous dose of tetanus toxoid-containing vaccine
- Pertussis-containing vaccines; : known anaphylactic reactions, encephalopathy within 7 days following previous vaccination, progressive or unstable neurologic disorder (e.g., coma, uncontrolled seizures, or progressive encephalopathy) 
Special patient groups
- See “Preconception counseling” for vaccination recommendations for women wishing to become pregnant.
- Preterm infants should receive all recommended vaccinations according to chronological age unless contraindicated.
Vaccinations in HIV-infected individuals
- Efficacy of immunization is reduced in HIV-infected individuals (due to impaired immune function)
The should be observed with the following exceptions:
- Live-attenuated varicella zoster and MMR vaccine should not be given if the CD4 count is < 200 cells/mm3 (CD4 percentage < 15% in patients ≤ 5 years) or if the patient has AIDS-defining conditions.
- Live-attenuated influenza vaccine is not recommended, regardless of the CD4 count; inactivated influenza vaccine should be given instead.
- The inactivated polio vaccine should be used instead of the live-attenuated polio vaccine.
- Immunizations that are not part of the standard immunization schedule:
- One-Minute Telegram 9-2020-1/3: SARS-CoV-2 vaccine candidate shows promising results in phase 1–2 trial
Interested in the newest medical research, distilled down to just one minute? Sign up for the One-Minute Telegram in “Tips and links” below.