Each of the interventions used alone reduces transmission of the infection, although they reduce transmission by different amounts and differ in the way they affect the dynamics of the local epidemic. Each intervention has its greatest effect when the reproduction number R is close to 1 (say R<1.5), because the total number infected reduced dramatically as R decreases from 1.5 to 1. This effect is similar to the effect known as herd immunity.

Isolating cases

For influenza, isolating diagnosed cases as soon as possible after their diagnosis is not likely to reduce transmission effectively when used by itself. This is seen by both its effect on R and its effect on the eventual number infected. This is because for influenza substantial transmission occurs before isolation can be effected in practice. However, if other interventions are able to reduce R to 1.5 then adding isolation of cases to these interventions will have a worthwhile effect and will possibly bring R below 1, to achieve elimination.
Isolating cases would be an effective intervention if the disease characteristics of the emerged pandemic strain include a long period of high infectivity following the onset of symptoms.

Personal infection control and distancing

The effect of reducing transmission by promoting behaviour that reduces close contacts between individuals depends on the extent to which contacts are reduced. However, the percentage by which contacts can be reduced is potentially higher than the percentage by which isolating cases can reduce the portion of the infectious period spent in the community. As a result, and noting that we get a double effect when reducing contacts (because both susceptible and infectious individuals avoid contacts), personal infection control and distancing is potentially quite effective, in terms of reducing R and the total number of cases. Furthermore, if an intervention consisting of isolating cases and an intervention consisting of personal distancing reduce R by the same amount, then distancing is preferable because, compared with isolating cases, it produces a flatter local epidemic, which is less disruptive and results in a lower peak burden on the health care system.

Closing schools

Using mixing rates for school children and adults that are consistent with group-specific influenza attack rates observed for school children and adults in previous pandemics of influenza it is found that closing schools has at best a modest benefit on overall transmission although it can assist in reducing the attack rate in children (see Section 4.4 for a discussion of assumptions and findings). If it turns out that the elderly are at greatest risk from the pandemic influenza strain, and closing schools has the effect of increasing the mixing between children and the elderly, then the value of closing schools becomes highly questionable. As with all interventions, closing schools has an appreciable effect when R is in the range 1–1.5.

Restricting travel within Australia

Levels of interstate travel are now very high and it is necessary to achieve very high levels of restrictions in movements to see any appreciable delay in the time before the infection moves from one state/territory to another. This is especially true for movement of the infection between state or territory capitals. Isolating a more remote region might be feasible if contact with infected regions can be kept to a very small number.

Quarantining households

Quarantining household members of a diagnosed case separates newly-infected individuals and potentially-infected individuals from the community before they become infectious or earlier in their infectious period. As expected, it is found to have considerably more potential to reduce transmission than merely isolating diagnosed cases, because infected household members have a considerably reduced potential to infect others. The cost is that some individuals who are not infected are also quarantined. The effect of quarantining depends considerably on the level of compliance, and results suggest that 60% compliance can produce a significant impact on transmission, case numbers and possible containment.

Use of antiviral drugs for limiting transmission

Under the default estimates of effectiveness for antiviral drugs calculations suggest that their use for targeted prophylaxis can delay the largest part of the infection incidence by several months. The default estimates of parameters suggest that the current stockpile of antiviral drugs is likely to delay the peak of the epidemic by one year, provided 40-50% of the exposed individuals are reached soon after the source case is symptomatic and the reproduction number is relatively low. As will be discussed in Section 5.6, this strategy is most effective when combined with treatment of symptomatic cases (see Figure 5.8).

Combined interventions

When interventions are used in combination, and assuming that

a. isolating cases reduces the part of the infectious period spent in the community by 20%,

b. personal infection control and distancing reduces the rate of making infectious contacts between susceptible and infectious individuals by 50%,

c. quarantining with 80% compliance, and

d. AVs are used for prophylaxis with 50% coverage,

then we are theoretically able to achieve elimination of the infection even if R0 is as high 10. To achieve those levels of intervention in practice would require a great deal of community co-operation.

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