Herd immunity

Certain types of infechous agent (e.g. influenza virus) are able to combat herd immunity sueh as this through undergoing major antigenic changes. These render the majority of the population susceptible, and their occurrence is often aeeompanied by spread of the disease aeross the entire globe (pandemies). 

Siebenga et ah, 2007b Zheng et ah, 2010). In each year, a novel strain was seen to circulate, the number of NoV outbreaks increased to atypical levels in many coimtries simultaneously (Johansen et ah, 2008 Lopman et ah, 2004 Siebenga etah, 2010). The testing of archived patient sera supports a hypothesis where herd immunity is acquired at the community level to an existing Gn.4 strain, reducing the number and size of outbreaks in years without novel variants (Cannon et ah, 2009). The detection of a new variant strain in the summer has been proposed as a predictor for winter epidemic seasons of NoV infection (Verhoef et ah, 2008). 

Herd immunity refers to high levels of immunization in one population resulting in protection of another unvaccinated population. For example, concentrated vaccination of children with the 7-valent pneumococcal conjugate vaccine resulted in decreased invasive Streptococcus pneumoniae infection not only in the vaccinated children, but also in elderly persons within the same community. 

Vaccinations can also confer enormous benefits to populations. When vaccination rates are sufficiently high they can create "herd immunity," a condition that occurs when the rate of susceptibility within a population is so low it interrupts the chain of transmission of a disease within a community (Board of Health Care Services 2004). "Herd immunity" is a classic public good it benefits all individuals within a population. 

Starr JM, Rogers TR, Impallomeni M. Hospital-acquired Clostridium difficile diarrhoea and herd immunity. Lancet 1997 349(9049) 426-8. 

This attitude not only exposes individual patients to unjustifiable risks, but also jeopardizes herd immunity, thus representing a threat to pubhc health. A homeopathic remedy might be totally safe, but the homeopath might not be (46). 

Vaccines have an exemplary safety record, with a benefit that is immeasurable. With today s risk-averse society and heightened awareness, vaccine safety attracts intense scrutiny, particularly in the case of preventative vaccines, which are given to healthy adults, infants, and children. The safety benefit ratio is skewed heavily toward safety and as many infectious diseases are not prevalent today, many people may not perceive the benefit. In fact, in recent years, there has been a shift toward a risk benefit for the individual, and as a result of certain alleged safety issues, there has been a reduction in vaccinations, (e.g., Japan and DTP vaccines, MMR in UK, polio in African regions) this will ultimately lead to decreased herd immunity, and the prevalence of certain diseases may return. Thus, the need to ensure safety is of utmost importance. It should be noted that the risk benefit may differ slightly for therapeutic vaccines. However, the emphasis on safety remains. 

Population characteristics Herd immunity resulting from vaccination programs or from infections endemic in the local community, together with other factors, such as social behavior, can affect the spread of infection in the community. 

Peterson, R. E., Klopfenstein, T. J., Moxley, R. A., Erickson, G. E., Hinkley, D., Rogan, D., and Smith, D. R. (2007c). Efficacy of dose regimen and observation of herd immunity from a vaccine against Escherichia coli 0157 H7 for feedlot cattle. J. Food Prot. 70, 2561-2567. 

Less than 100% protection may be sufficient to prevent the spread of the disease within the population, since the likelihood of a susceptible individual encountering an infected individual becomes reduced. This phenomenon is known as herd immunity and is an important consideration in assessing the potential of individuals in a population to succumb to infection. 

Many factors can affect the quantity and quality of an immune response, most of which have been mentioned previously. Other factors that affect the immune response are stress, pregnancy, surgery, concomitant infections, extremes of temperature, and especially malnutrition. All these factors reduce the quantity and quality of the immune response, and when the immunoassayist performs seroepidemiological surveys to assess the herd immunity among various populations, these factors must be taken into account prior to determining levels of adequate protection within a population. 

Active immunization Use of vaccines to control diseases by increasing herd immunity through stimulation ofthe immune response. 

Herd immunity The proportion of individuals in a population who are immune to a particular disease. 

This leads to what is called the prevention paradox—it is a common irony that many people must take precautions to help the few. ° Few of us have experienced severe load on our seat belts (no immediate personal gain), but because most of us wear belts all of the time, the minority who are involved in crashes are protected. In preventive medicine this is known as herd immunity. In the UK, Rose et al. estimated that for every 400 drivers belted, one life is saved. Forty years ago, 600 children had to be immunised against diphtheria for every life saved. If the risk reduction offered to an individual is small (as it is in both these examples), then the cost to the individual of that intervention must be correspondingly small. ... 

It is important to underline that governments had to mandate seat belt wearing in order to achieve herd immunity. For decades in Australia extensive public education... 

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