A vaccine is an immunobiological substance that is introduced into the body with the aim to confer specific immunity against a certain disease caused by microbes like bacteria and viruses. Vaccines are either made up of whole and weakened or a component of the disease-causing microorganism, which mimics its infectious activity after entering the body, thereby stimulating the immune system to respond by signalling B cells to disease-specific antibodies. These B cells can recognise the disease-causing pathogen in case it enters the body again to cause an infection and can suitably respond by producing specific antibodies. Depending on the type of vaccine, the mechanism defers but the principle of eliciting an immune response remains the same. Although in some vaccines, like live vaccines and killed vaccines, the actual disease-causing microorganism is introduced into the body in small doses, either after weakening or attenuation (live vaccine) or inactivation (killed vaccine), the risk of getting the infection is next to none. Vaccines are generally available in liquid form (or sometimes in powder form and have to be reconstituted with the appropriate fluid) and are typically injected or given as oral drops. Other means to give injections can include the nasal route. The method of vaccination depends on the type of vaccine and is generally not privy to changes.

Although the terms are often used interchangeably, immunisation and vaccination are, strictly speaking, not the same thing. Vaccination refers to the process of administration of a vaccine, whereas, immunisation is the body’s response to it as it develops protective antibodies and becomes immune to the pathogen. Immunisation is not exclusive to vaccines as preformed immunoglobulins (the medical term for antibodies) are used clinically for post-exposure prophylaxis or protection from infection after having been exposed. Immunoglobulins are also used intravenously in autoimmune disease flare-ups and given to babies at birth for added protection from certain infections like Hepatitis B. Additionally, it would be beneficial to note that as part of routine childhood immunisation, children receive not only vaccines against infectious diseases but also Vitamin A. Vitamin A is not a vaccine but it helps boost the body’s immune system response and create antibodies towards the vaccine, or immunogen, more efficiently. Additionally, to protect against the drastic, and sometimes fatal, consequences of vitamin A deficiency the World Health Organization (WHO) recommends that high dose vitamin A supplements be given together with routine vaccines to children between 6 months and 5 years of age. The reason the two activities are combined is to increase coverage and ease the logistical burden. Immunisation, or vaccination, programmes can be routine for target vulnerable populations (like children, elderly, pregnant women, etc.), in case of special medical conditions (like after splenectomy) or in the case of emergency situations like outbreaks, epidemics or pandemics.

(Read more: Vaccines for newborns, infants and children)

  1. Types of vaccines
  2. Indications for vaccination
  3. Contraindications for vaccination
  4. Precautions to take when getting vaccinated
  5. Preparation for vaccination
  6. Procedure for vaccination
  7. Adverse events following immunisation (AEFI)
  8. Result of immunisation

Following are some of the different types of vaccines used currently:

  • Live or attenuated vaccines: The disease-causing pathogen is cultured in a laboratory and weakened, or made less infectious (attenuated), to create live vaccines. The organism used is incapable of causing full-blown disease but is very highly immunogenic, which means it can harness the most potent immune response in the body. Examples of common live attenuated vaccines are:
  • Killed or inactivated vaccines: To create inactivated or killed vaccines, the pathogen is first grown or cultured in a laboratory and then killed or inactivated with a chemical. It is safer than live vaccines but is less immunogenic (makes fewer antibodies than live vaccines). These types of vaccines usually need a primary dose followed by a booster dose to elicit a full immune response. Examples include:
  • Protein vaccine or viral vector vaccine: In such vaccines, instead of introducing the entire organism into the body, a part of it is used. Usually, a protein component of the organism is introduced into the body and is enough to be recognised by the immune system as an invader and launch antibody production in response to it. This type of vaccine is very safe. Examples are:
  • Toxoid vaccines: Some organisms produce toxins that cause disease. These toxins are extracted and detoxified for use as vaccines. Examples are: 
  • Polysaccharide vaccines: Some bacteria have a protective shell made of a chemical called polysaccharides. Introducing only this bacterial component in the body is sufficient to produce antibodies. Examples of such vaccines are:
    • Polysaccharide pneumococcal vaccine
    • Polysaccharide meningococcal vaccine
  • Conjugate vaccines: Small children younger than two years of age do not respond well to vaccines like those made of polysaccharides as their body’s immune system is unable to recognise the chemical. For them, a special type of vaccine is used that fuses another chemical to the polysaccharide component allowing antibodies to form in children. Examples of conjugate vaccines are:
    • Conjugate pneumococcal vaccine
    • Haemophilus influenza B (Hib) vaccine
  • Recombinant vaccines: DNA recombinant technology allows the synthesis of proteins of the organism that can be used to elicit an immune response in the body. Some such vaccines are:

(Read more: How are vaccines made)

Following are some situations and conditions in which vaccination may be administered: 

  • National Immunisation Schedule for childhood immunisation: All children in India receive certain vaccines and their booster doses in a scheduled manner. 
  • Pregnant women: All women must receive two doses of tetanus (ideally Tdap) vaccine during pregnancy, the first dose in the first trimester followed by a second dose 4 to 8 weeks later. A tetanus booster is given to pregnant women if two doses of the tetanus vaccine were received during pregnancy in the last 3 years. Other vaccines may be needed based on special circumstances.
  • Human papillomavirus (HPV) vaccines for young women: HPV infections are common in young females, especially after becoming sexually active. Although the infections is self-limiting, certain strains of the virus can lead to cervical cancer, which is preventable with timely vaccinations. In India, two doses are given to girls between 9 and 14 years of age, and three doses to those over the age of 15. If no vaccine has been received previously, it can be administered between the age of 13 and 45. 
  • Immunodeficiency like HIV-AIDS: HIV positive patients have weak immune systems and are prone to vaccine-preventable opportunistic infections (OI) from specific pathogens. 
  • Splenectomy patients: Patients who undergo a splenectomy become more vulnerable to invasive infections by encapsulated bacteria. It is recommended they receive pneumococcal, meningococcal and haemophilus influenza B vaccine, either two weeks before (in case of elective surgery) or two weeks after (in case of emergency surgery) the surgery. Additionally, they must get the influenza vaccine every year at the start of the flu season in winter.
  • Elderly population: As older people (usually over the age of 65) have a weaker immune system, certain annual vaccines are advised to them.
  • International travel: Countries require travellers to have evidence of vaccination against certain diseases prevalent in the destination of their departure to prevent spread. For example, yellow fever vaccination is required for all travellers entering India from endemic areas.
  • Outbreaks, epidemics and pandemics 

(Read more: Pandemic v/s epidemic)

Vaccination is a safe, essential and potentially life-saving measure against preventable infectious diseases and even some cancers. However, in the presence of certain factors, vaccination should be avoided or delayed. Many contraindications are temporary and only warrant delaying the dose. Absolute contraindications to vaccination include:

  • History of severe allergic reaction or anaphylaxis to the previous dose of the vaccine or any of its constituents
  • Severely immunocompromised patients (including HIV) should not receive live attenuated vaccines
  • Pregnant women should not receive live attenuated vaccines due to risk to the fetus
  • A history of encephalopathy within seven days of a pertussis-containing vaccine is a contraindication for any future pertussis vaccines
  • Severe Combined Immunodeficiency (SCID) disease and a history of intussusception are contraindications to the rotavirus vaccine
  • History of wheezing or a diagnosis of asthma in children between 2 to 4 years, aspirin treatment and oseltamivir (Tamiflu) dose in the last 48 hours are contraindications to influenza vaccines
  • Covid-19 vaccines (Covishield and Covaxin) may not be administered in case of the following factors:
    • History of allergy to COVID-19 vaccine or any of its constituents
    • Age under 18 years
    • Pregnant woman
    • Breastfeeding or lactating woman
    • Delay the vaccination by 4 to 8 weeks in case of active COVID-19 symptoms
    • Delay the vaccination by 4 to 8 weeks in case of having received COVID-19 convalescent plasma
    • Delay the vaccination by 4 to 8 weeks in case of admission to intensive care unit (ICU) for any illness

Vaccines are safe and effective means to safeguard from preventable infectious diseases, but certain temporary factors can warrant precautions to be taken and doses to be delayed if necessary. Precautions to take with vaccines include:

  • The presence of a moderate or severe acute illness with or without a fever is a precaution to the administration of all vaccines. Depending on the severity of symptoms the vaccine may be administered or delayed. It helps avoid diagnostic confusion between manifestations of the underlying illness and possible adverse effects of vaccination.
  • Present, recent or upcoming anesthesia/surgery/hospitalization may need vaccination to be deferred as the immune system is weaker at this time.
  • A personal or family history of seizures is a precaution for MMR (measles mumps rubella) and varicella vaccination.

Vaccinations do not need any special preparatory measures but the following steps can make the process smoother.

  • Wear clothing that allows access to the upper arm and thighs if possible. 
  • If scared of needles and prone to vasovagal fainting, it is advisable to not go on an empty stomach. A light meal can help.
  • Avoid caffeine if prone to palpitations. 
  • Do not take antipyretic (fever relieving) medicine beforehand in anticipation of symptoms following vaccination.

Before beginning the process, the vaccinator will ask questions about concurrent fever or any recent symptoms. A history of previous allergic reactions to vaccines will also be taken. The vial of the vaccine being used will be shown to the patient and information will be provided about the vaccine, possible side effects and what to do in case they arise. After receiving the patient’s informed consent, the vaccinator usually cleans the site to be injected with a small alcohol swab. Common sites for injections are the upper arms, sides of thighs and buttocks. Depending on the type of the vaccine, it may be injected intradermally, subcutaneously or intramuscularly. The syringe needle is positioned along the skin at a zero degree incline in case of intradermal injections, at 45 degrees in case of subcutaneous injections and at a 90-degree angle for intramuscular. Slight bleeding (a drop or two of blood) is possible and a cotton swab is provided to be applied with pressure at the site. After receiving the vaccine the patients are asked to wait in an observation area for at least thirty minutes in case an adverse event following immunisation were to arise.

(Read more: What are mRNA vaccines)

Experiencing fever or body ache after a vaccination session is normal and paracetamol or ibuprofen may be taken for it. In case of ache at the site of injection, analgesics can be taken or a clean and cool cloth can be used to relieve pain.

Even though vaccines are extremely safe, some complications are possible with their use. Adverse events following immunisations (AEFI) is any untoward medical reaction that occurs after immunisation that may be expected with the vaccine, unexpected and harmful or even incidental. While each vaccine is associated with its own set of expected potential side effects, like fever or body ache, other possible complications following vaccination are as follows:

More serious complications include: 

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