A What is herd immunity?
Herd immunity most often refers to the protection of non-immune individuals as a result of being surrounded by immune individuals. High levels of immunity make it less likely that a susceptible individual will come into contact with someone who is shedding or colonised with the pathogen, thereby offering indirect protection.
Which infections show this herd effect?
For herd or indirect effects to be observed, immunity from vaccination or past infection must impact on transmission of infection. Herd effects are observed for many recommended vaccines including measles, mumps, rubella, pertussis, poliomyelitis, varicella, Haemophilus influenza type B, meningococcus and pneumococcus.
In fact, much of the impact of childhood vaccines used today result from these indirect effects.
At what point are herd effects observed?
Even in regions with modest uptake (<50%) of conjugate Haemophilus influenzae type B vaccination in children, a reduction in disease was observed in unvaccinated children, demonstrating the significant impact of these vaccines on nasopharyngeal colonisation and disease.
In Australia and other countries, dramatic changes in pneumococcal disease have been observed in all age groups following introduction of conjugate pneumococcal vaccination in young children.
What proportion of kids must be vaccinated to prevent outbreaks?
There is an inverse relationship between the herd immunity threshold (the level of immunity required to prevent outbreaks) and basic reproduction number (Ro: the number of secondary cases generated by a typical infectious individual in a randomly mixing susceptible population).
In other words, the more infectious the agent, the higher vaccination rates need to be to prevent infection. Measles and pertussis (both have Ro > 12) require immunity rates to be as high as 95% to achieve sufficient herd immunity to prevent transmission. Less infectious agents (e.g. rubella, Ro ≈ 6-12; polio, Ro ≈ 5-7) requires lower rates of community immunity (approximately 85%), to prevention transmission.
Fine P, Eames K, Heymann DL. “Herd Immunity”: A Rough Guide. Clinical Infectious Diseases, 2011, 52 (7): 911-916. doi: 10.1093/cid/cir007
Plans-Rubio P. Evaluation of the establishment of herd immunity in the population by means of serological surveys and vaccination coverage. Hum Vaccin Immunother. 2012 Feb;8(2):184-8. doi: 10.4161/hv.18444.