by Srivallabh Sabnivisu
Viruses are small, in fact, smaller than bacteria. They have simple compositions and exist in different shapes. They cannot multiply without a host cell, which is usually plants, animals, and bacteria. They are not visible under a light microscope. They are alive, highly contagious, and reproduce quickly. Unlike bacteria, they cannot be grown on artificial media. The virus was first discovered in 1892 by Dmitry I. Ivanovsky and Martinus W Beijernick. Since the 1960s, improved techniques like Electron Microscopy, electrophoresis on gel substrates, X-ray diffraction, and other molecular biological techniques gave a better insight into the structure, function, and composition of a virus.
When a pathogen enters the body of the host, it creates an immune response. Fever and inflammation are some of the defensive mechanisms of the body to kill or fight against the pathogen. The immune system of humans can be divided into two types: Innate or non-specific immunity through phagocytes and Specific immunity through the production of antibodies by WBC (T and B-cells).
Administration of vaccines provides controlled exposure to the specific pathogen so that the immune system of the host is strengthened by producing memory T and B cells to fight against it specifically. If the body encounters the same pathogen in the future, the memory cells that “remember” the pathogen help in fighting against it more efficiently and rapidly.
Figure(1) Immune response upon administration of a vaccine. (Ref(1))
Vaccines can be administered through different routes. They include mouth (oral), into the fatty tissue beneath the skin (subcutaneous), into muscle tissue (intramuscular), into layers of skin (intradermal) or through nose (intranasal).
Although vaccines provide effective immune system strengthening, due to differences in the individual immune systems, there might be differences in the immune response. So, some vaccines require more than one dose in order to generate adequate response required by the individual, ex: Polio vaccine.
Type of vaccine | Definition | Effective against |
Live, attenuated | Weakened or altered so that it cannot cause illness | Measles, Mumps and Rubella (MMR) vaccine, Varicella (chickenpox), Influenza and Rotavirus, Zoster (Shingles), Yellow fever. |
Inactivated/Killed | Pathogen inactivated using heat or chemicals like formalin or formaldehyde so that it cannot replicate but is still intact. | Polio, Hepatitis A, Rabies. |
Toxoid (Inactivated Toxin) | Vaccines created using inactivated toxins of the pathogen. | Diphtheria, Tetanus (part of DPT vaccine). |
Subunit/Conjugate | Contain only portions of the pathogen, like specific protein from a pathogen. Recombinant vaccines also belong to this type. | Hepatitis B, Influenza type b (Hib), Pertussis (part of DPT vaccine), Pneumococcal vaccine, Meningococcal vaccine, HPV. |
Table (1) Types of vaccines and the diseases they are used against. (Ref(2))
I am going to discuss more about MMR vaccine given to children in UK between 1 and 3.5 years. It was introduced in UK in 1988. MMR vaccine is a single injection that protects children against three pathogens that cause Measles, Mumps and Rubella. Measles is caused by the virus Rubeola, Mumps is caused by Paramyxo viruses while Rubella is caused by virus named Rubella. All three are highly infectious diseases which could lead to meningitis, encephalitis and deafness.
This vaccine is administered to the baby in two doses, one at 13 months and the other at 40 months as a live, attenuated form of vaccine (2). Antibodies for these viruses are passed on from mother to baby at the time of birth and therefore it is usually not advised to give the vaccine to the baby before they are twelve months old. The reason behind this is those antibodies can fight against the vaccine and make it ineffective. These antibodies are gradually lost by the time they reach 12 months. It is administered as an intramuscular injection. Given the nature of the vaccine, side-effects are very rare. But sometimes a mild infection of measles or mumps could be seen but will last only for 2-3 days and it is also to be noted that it is not infectious when experienced after taking a vaccine. However, a single vaccine or six separate doses of the vaccine are not recommended due to higher risk of side effects and infection. In case of any doubt regarding second dose of vaccine, it is always advised to have another dose to make sure the child is adequately protected.
The immune response to Measles and Rubella is long lasting, whilst there is a chance that the immunity to Mumps might decrease over time. However, it is highly unlikely to develop serious complications once two doses of vaccine are taken during childhood. It is always recommended to keep up to date with this vaccine, in case of untimely outbreak of Measles.
(1) Wim Jiskoot, Gideon F. A. Kersten, Enrico Mastrobattista, Bram Slütter, “Vaccines”, Pharmaceutical Biotechnology (2019): 281-304.