CONTROL OF MICROBIAL INFECTIONS
One of the values of Ro lies in what its composition tells us. The characteristics that comprise Ro
must be essential and important for the organism, therefore reduce any of these values and the efficiency of spread will be impaired. If any of the parameters are reduced to zero, in theory the epidemic will be halted. If then the number of susceptibles (S), the efficiency of transmission (β) and infectious period (D) will all be suitable targets for control of the epidemic. The easiest target will often be to reduce the number of susceptibles through vaccination. As a marker of the ability of an organism to spread, Ro gives an indication of the propensity of an organism to invade a population (cause an epidemic). Organisms of high Ro will spread very efficiently and infect the very young. Ro can be used to predict the levels of vaccination needed to eradicate infectious disease. The effective reproductive rate R must be less than one for the disease to disappear.
Earlier, we defined , where xis the fraction of the population susceptible. If a completely susceptible
population is represented as unity, then the actual fraction of the population immunised can be
represented as (1^-P), hence:
Substituting R with 1 yields:
This can be rearranged to find the percentage (p) that need to be vaccinated as follows:
1/ Ro = 1^-P
1/ Ro + P = 1
P= 1- 1/ Ro
p will be the boundary condition, i.e. it sets the minimum proportion of the population that needs to be
vaccinated in order for the effective reproduction rate to fall to one. It follows that vaccination rates
must not be less than that set by 1- 1/ Ro.
To combat measles it is necessary to achieve very high vaccination rates (around 95 per cent uptake) in children as young as possible. It is clearly not that helpful to vaccinate children at an age that is older than the average age of infection.
What measures can be implemented to reduce βand D?For highly infectious diseases such as measles the idea of mass action as the basis for describing the random mixing of infectious and susceptible people appears to be a good model for describing what actually happens. Hence, Ro
proves useful in comparing the infectiousness of different organisms. With measles, diphtheria and rubella, each confer lifelong immunity. This characteristic helps in monitoring a population for immunity as the protective antibodies, all developed against a single dominant antigenic type, can readily be measured in the serum of people. The variation in antigenic types seen with influenza virus make predictions of the virus almost impossible, as well as the manufacture of correct vaccine that will be protective every year. Ro can be estimated from serological surveys of the population. The age at which antibodies appear can be used to calculate Ro (the younger the age at which the
infection occurs equates to a higher Ro and vice versa). Again, such methods are problematic for
influenza virus because different strains of influenza virus may circulate through the population
periodically, making the serological testing complicated. Not all infectious diseases spread horizontally like measles. Hepatitis B virus is acquired parenterally or vertically. Such transmission is not random; the chances of all individuals in the population encountering each other are not equal, thus mass-action models do not apply. An additional difficulty is that the efficiency of transmitting the infection may also vary between different groups of people. Sexually transmitted diseases are an example of a non-homogenous mixing of the population (celibate people will not become infected) as well as examples of differences in the efficiency of transmission (transmission of HIV and Hepatitis B virus is less efficient in heterosexual partners than in homosexuals).
What measures exist to prevent and reduce the incidence of infection? Arguably the provision of clean drinking water, nutrition, sanitation and general economic health have a profound effect, but to these vaccination must be added. Only vaccination has resulted in an infectious disease being eradicated from the human population, with the global vaccination against smallpox declared successful by the World Health Organisation in 1979. The mechanisms by which vaccination works in humans is essentially immunology and will not be covered here. The area of interest to us is what vaccination tells us about infectious diseases of humans. The success of vaccination depends foremost on our understanding the physiology and ecology of the organism in question.
Whilst vaccination protects the individual, there are benefits to the whole community when vaccination uptake rates are high. The few remaining individuals that have not been vaccinated are protected by the herd immunity principle.
As discussed previously, DT represents the minimal number of susceptibles that are needed for a particular organism. Herd immunity arises if the number of susceptibles fall below that value. Herd immunity in humans only applies to infectious diseases that are strict human pathogens. Any
infection that is acquired from the environment or from animals will be unaffected by the reduction in
susceptibles (DT ) because it has a natural reservoir distinct and separate from humans (i.e. the
environment or animals). For example, the reservoir for tetanus bacilli is the soil and rabies virus is a
zoonosis. Neither are caught from other humans. Thus, vaccination of other people provides no
protection to the individual who has caught tetanus from a rose bush thorn or rabies from an infected
dog. Note how influenza virus epidemics cloud the distinction. Influenza originates from an avian
reservoir (notably ducks) and once transferred to humans, spreads as epidemic influenza from person to person.
Strictly, this qualifies influenza as a zoonosis. The vaccination coverage is selective, i.e. targeted
to the most susceptible, such as elderly people with chest disease. In this circumstance vaccination rates are unlikely to ever impact on the DT , hence effective reproductive rate (R) is unaffected and herd immunity is unlikely to occur. Ideally, a vaccine should be stable enough to withstand being transported across the country without loss of potency. One of the reasons for the success of the smallpox vaccine was its stability at high temperatures whilst being delivered to the populations close to the Equator. Also, a vaccine should be totally safe and effective, cheap and easy to administer (preferably oral) and provide lifelong immunity. The reality is that some vaccines have many of these features but none possess them all. Despite this vaccines have reduced the incidence of infectious disease, reduced the incidence of lifelong sequelae and disability and reduced the mortality rate.
DDD, the value of vaccines:
• Reduce Disease,
• Reduce Disability,
• Reduce Death.
The types of vaccine that are produced commercially can be separated according to whether the vaccine uses intact organisms, either live or killed, or components/ products of organisms. What follows are selected examples and important features of the different types of vaccine currently used.
Cited By Kamal Singh Khadka
Msc Microbiology, TU.
Assistant Professor In PU, PBPC, PNC, LA, NA.
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