1. Dr Dipesh Tamrakar 1st yr JR SPH & CM

2. Content Definition Basic concepts Beneficial and Deleterious effect Example Recent concepts 3. Herd Immunity Oxford Text book of public health It is the relative protection of a population group achieved by reducing or breaking the chains of transmission of an infectious agent because most of the population is resistant to infection through immunization or prior natural infection 4. Definition contd Harrision principle of internal medicine`` Successful vaccination protects immunized individuals from infection, thereby decreasing the percentage of susceptible persons within a population and reducing the possibility of infection transmission to others. At a definable prevalence of immunity, an infectious organism can no longer circulate freely among the remaining susceptibles. This indirect protection of unvaccinated (nonimmune) persons is called the herd immunity effect 5. Herd Immunity and Herd Effect Herd Immunity:``the proportion of subjects withimmunity in a given population. Herd Effect:``the reduction of infection or disease in the unimmunized segment as a result of immunising a proportion of the population''T. Jacob John & Reuben Samuel 6. Herd Immunity Vs Herd Protection Herd protection: Protection to the unimmunizedindividual without inducing immunity, virtually by breaking the transmission of the infection or lessening the chances of susceptible coming in contact with infective individual Herd immunity: Immunity to the unimmunized individual by secondary spread of the attenuated viruses or bacteria in the vaccine when shed in fecal matter.Yash Paul 7. 1. Herd immunity and Herd protectionoral poliovirus vaccine (OPV), oral typhoid, and oral rotavirus. 2. Herd protection only inactivated poliovirus vaccine (IPV), diphtheria, pertussis, measles, mumps, rubella, varicella, pneumococcal, meningococcal, hepatitis A, hepatitis B, typhoid (other than oral). 3. Do not provide any additional benefit to the unimmunizedtetanus, rabies, and Japanese encephalitis.Yash paul 8. Concepts Behind Herd Immunity 9. Contd 10. Contd 11. Basic Reproductive Number The basic reproductive number (R0) is the averagenumber an infectious person will infect with an agent in a completely susceptible population.Peter G Smith 12. ContdPeter G Smith 13. ContdPeter G Smith 14. ContdPeter G Smith 15. ContdPeter G Smith 16. Peter G Smith 17. Herd immunity threshold: Basic concepts In order for an infection, to be maintained in a population, each case of infection must give rise to at least one other case. The effective reproduction number (R) has to be reduced below 1. If a proportion P of the population are immune then R = (1- P) R0 So, to get R down to about 1, P must be up to 1-1/ R0 Thus if R0 = 5 then vaccine coverage will have to be in excess of 80% The value of P that reduces R to at most 1 is commonly called the herd immunity threshold Peter G Smith 18. Fine et al 19. (Approximate) Herd Immunity Thresholds for Infection EliminationPeter G Smith 20. Basic Reproductive Number R0 will vary from agent to agent depending on theinfectiousness of the agent e.g. -how long it survives in the environment - the dose necessary for infection - the duration of infectiousness in the host -Whether or not infectiousness precedes infection symptoms R0 may vary between different populations and in different segments, or at different times, in the same population 21. Transmission dynamics of an infectious agent and Herd immunity Infections for which herd immunity may be importantin reducing the risk of infection in non-immunes in the population are those : which are transmitted directly from person to person (e.g. measles, rubella, varicella) for which humans are the reservoir, or an important reservoir of infection (e.g. polio, malaria) There may be no herd protection if human are not an important reservoir of infection (e.g. tetanus, rabies 22. Beneficial effects Potential for infection elimination Reduced risk of infection for those refusing vaccination (free riders) Reduced risk of infection for those for who vaccination is contraindicated (e.g.immunosuppressed) 23. Deleterious effects Raise the average age of infection among those who are infected Particular problem for those infections where the severity of infection increases with age (e.g. polio, rubella, mumps varicella, measles, hepatitis A) 24. Example 25. Pneumococcal vaccine A seven-valent pneumococcal conjugate vaccine (PCV7)was introduced to the routine childhood immunization in the USA in 2000. Evidence suggests that the elderly have indirectly benefited considerably from this introduction of conjugate vaccine in US children. Example :comparing prevaccination and postvaccination eras, showed that cases of IPD, IPD deaths and hospitalizations related to pneumonia all decreased substantially with a 54% reduction in nonbacteremic pneumococcal pneumonia among adults 65 years of age or older 26. Meningococcal In Canada, following introduction of a meningococcalC vaccine program in infants and adolescents under 20 years of age, a trend of decreasing serogroup C incidence was also observed among individuals aged20 years or older [incidence rate ratio (IRR) per year 0.84, 95% confidence interval (CI) 0.740.95]; incidence decreased by 16% per year, on average, despite the fact that this group was not the target of the provinces conjugate C program 27. Rota virus Austria was the first European country to implementuniversal mass vaccination against RVGE for all infants nationwide Epidemiological data from a hospital-based surveillance system in Austria show that incidence rates of children hospitalized with RVGE decreased in 2009 compared with 2008 and compared with the prevaccination period 2001 2005. Decreasing hospitalization rates from RVGE were observed in children of all age groups, even in those not eligible for vaccination according to their age, suggesting either a secular trend and/or herd immunity induced by universal mass vaccination against RVGE 28. influenza a cluster randomized trial among Hutterite colonies inCanada : found that the rate of PCR-confirmed influenza was significantly lower in the influenza vaccine colonies compared with the control colonies with an overall protective effectiveness against influenza of about 60% in contacts of influenza vaccinated children, suggesting a substantial herd effect of the vaccine In a recent US trial involving school children aged 411 years, herd protection attributed to LAIV was detected for essentially all age groups despite a vaccine mismatch 29. Varicella the Australian vaccination strategy targeted children aged18 months together with a catch-up campaign for children aged 1013 years of age with no history of prior varicella infection or vaccination An Australian study, comparing the incidence of congenital and neonatal varicella during the prevaccination and postvaccination eras, found a 100% reduction in congenital varicella and 85% reduction in neonatal varicella since the vaccination was implemented in 2005 So, the apparent reduction in the numbers of congenital and neonatal varicella in the unvaccinated age groups appears a clear indication of vaccine herd effects. 30. Measles Measles is a highly contagious disease (R0 estimated tobe >10) that is preventable by immunization but requires a very high vaccine uptake to maintain herd immunity The need for maintenance of this high coverage to stop transmission is illustrated by recent outbreaks in Europe and elsewhere following a misleading vaccine safety concern there has been increased outbreaks of measles, in Europe, with more than 26 000 cases of measles reported, with 14 000 in France alone 31. Potential herd effects in HPV vaccination The herd effect of HPV is not yet known The direct effect was supported by a meta-analysis of 6randomized controlled studies, showing a reduced frequency of high-grade cervical lesion by an OR of 0.14 with the high efficacy of the vaccine against cervical cancer, a vaccine herd effect might be expected, especially in high endemic regions the effect of a vaccine herd effect in women by vaccinating men, or vice versa T. H. Kim et al. 32. Herd effect of interventions other than vaccination lymphatic filariasis: , mass treatment with Ivermectinor diethyl carbamazine - the drug causes a reduction in transmission to susceptible even if they had not taken the drug Pulmonary Tuberculosis: If increasing proportions of persons with pulmonary tuberculosis are diagnosed and treated early, the incidence of infection in the susceptible population should continue to decline 33. TheoreticalDevelopment: Recent concepts 34. Imperfect Immunity vaccination does not confer solid immunity againstinfection to all recipients, (Example: multiple doses and monovalent vaccines in india) the threshold level of vaccination required to protect a population increases Vc=(1-1/R0)/E( Vc= critical vaccine coverage and E= vaccine effectiveness against infection transmission if E is