Immunity is a state of resistance to a disease through the defence activities of the immune system. It is of two types: innate or natural and acquired.

A.  Natural immunity to a disease, which is species specific, whereby different species of animal kingdom suffer from different diseases and are resistant to some diseases. Similarly some physiological conditions influence vulnerability or resistance to some diseases. It has been observed that people with sickle cell disease are resistant to malaria and individuals with Fibrocystic disease do not develop pulmonary tuberculosis, though may develop tuberculosis of other organs or systems.

B.  Acquired immunity can be acquired passively or actively.

  (i)  Passively acquired immunity:
        Maternal antibodies provide protection against some diseases during early childhood and administration of immunoglobulins provides instant but short lived immunity.

  (ii)  Active immunity occurs following :

              (a)  Infections. In general viral infections result in long lasting immunity, thus second attack by the same virus occurs rarely, but bacterial infections do not provide such benefit, as subsequent attacks by same bacteria are known to occur e.g., recurrence of typhoid fever, or tonsillitis or otitis.

              (b) Vaccination. Vaccines are administered to generate antibodies against specific infection(s). In general vaccines against virus infections cause long lasting immunity.

              (c) Secondary spread of live vaccines. In case some live vaccine reaches close contact, it may generate antibodies in the contact. Thus this person gets benefit of immunization without taking the vaccine.

"Just as individual immunity decreases the probability of an individual developing a particular disease when exposed to an infectious agent, herd immunity indicates decreased probability of a group or community developing an epidemic upon the introduction of an infectious agent, although there may be certain number of persons who are individually susceptible to the agent, thus an entire population (100%) does not have to be immunized to prevent occurrence of an epidemic. When most of the population in a community are immune to a particular infection that is spread from person to person, the natural transmission of the infection is effectively inhibited(1)".

"The herd immunity may be considered as being dependant on resistance to attacks by a disease to which a large population of the individual members are immune, thus lessening the likelihood of an individual with the disease coming into contact with a susceptible individual"(2).

The immunized person may provide two benefits to the community, namely: (i) herd immunity; and (ii) herd protection.

Herd immunity: Immunity to the unimmunized individuals by secondary spread of the attenuated viruses or bacteria in the vaccine. There are 3 documented cases where varicella vaccine had resulted in immunity in close contacts(3, 4). Earlier it was thought that OPV provides additional benefit to the community by secondary spread of attenuated polioviruses. Now it is known that this additional benefit which may occur could be the boosting effect on the already existing immunity. The additional benefit of herd immunity does not occur because of two reasons: (i) attenuated polioviruses contained in OPV have markedly reduced infectivity, and (ii) low load of vaccine viruses spread through feces. There are about 1,000,000 type 1 polioviruses, about 100,000 type 2 polioviruses and about 600,000 type 3 polioviruses ie, about 17 lakh polioviruses in each dose of two drops of OPV. On the other hand, one gram of fecal matter of vaccine recipient contains about 100 vaccine polioviruses (5). Thus, 17 kg of fecal matter may provide same quantity of vaccine poliorivuses as are contained in one dose of OPV. How much antibodies would be generated by few thousand vaccine polioviruses spread through feces when many doses of OPV, each dose containing about 17 lakh vaccine polioviruses have failed to generate protective immunity in many children?

Herd protection: Protection to the unimmunized individual without inducing immunity, virtually by breaking the transmission of the infection or lessening the chances of susceptible coming in contact with infective individual(6).

            In clinical practice, herd immunity does not play significant role, while herd protection plays a major role, though to a limited extent, because unimmunized individuals do not develop immunity, but enjoy the protection because of break in spread of infection. Thus, the herd protection is the major beneficial component of immunization for unimmunized population for the infections which spread from person to person(6). It should be remembered that these unimmunized individuals enjoy protection till they are among immunized and resistant people. As they have not developed immunity, may develop disease if come in direct contact with    infected person, in case they shift to the milieu where there is outbreak of the disease.

Depending upon whether the additional benefit(s) being provided or not, the vaccines may be placed in three groups:

1. Vaccines which can provide herd immunity and herd protection - oral poliovirus vaccine (OPV) varicella vaccine, any live vaccine (theoretically possible).

2. Vaccines which can provide herd protection only inactivated poliovirus vaccine (IPV), diphtheria, pertussis, measles, mumps, rubella, pneumococcal, H-influenzae b (Hib), meningococcal, hepatitis A, typhoid and BCG.

3. Vaccines which do not provide any additional benefit to the unimmunized persons - tetanus, rabies. Japanese encephalitis, hepatitis B.

-Yash Paul
Jaipur-302 016, India.
E-mail : dryashpaul2003@yahoo.com

REFERENCES

1.  Collee JG. Prophy lactic immunization. In: Greenwood D, Slack RCB, Peutherer JF, editors. Medical Microbiology, 14th ed. Edinburgh: Churchill Livingstone: 1992 pp. 791 - 801.

2. Miles A. Herd infection and herd immunity. In: Wilson G, Dick HM, editors. Topley and Wilson's principles of bacteriology, virology and immunity Vol. 1, 7th ed. London: Edward Arnold; 1953, p. 413.

3.  Chartrand S. Varicella Vaccine. Pediatr Clin North Am 2000; 47: 373 - 394.

4.  Takahashi M, Otsuka T, Okuna Y, et al. Live Vaccine Used To Prevent the Spread of Varicella in Children in Hospital. Lancet 1974; 2: 1288 - 1290.

5. Chen RT, Hausinger S, Dajani AS, Hanfling M, Baughman AL, Pallansch MA, et al. Seroprevalence of antibodies against polioviruses in inner city preschool children. JAMA 1996; 275: 1639-1645.

6. Paul Y. Herd immmunity and herd protection. Vaccine 2004; 22: 301 - 302.