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Berman et al. (1983) reported data on acute lower respiratory tract infections in children under 5 years of age attending ambulatory clinics in Colombia. A viral diagnosis was reported in 20 percent of cases: RSV was found in 9 percent and parainfluenza viruses in 2.1 percent. Serologic data reported by Monto and Johnson (1968) for three areas in Latin America suggest that the behavior and distribution of viral respiratory disease agents in the tropics are generally similar to those of the same agents in the temperate zones.

The data discussed above appear to be the best basis on which to estimate the disease burden proportion of noninfluenza ARI that can be attributed to the pathogens that are candidates for vaccine development. No direct information is available on the proportions of deaths due to the various pathogens incriminated in ARIs. To estimate deaths, it is therefore assumed that the proportion of deaths due to each agent parallels its isolation in lower respiratory tract illness/pneumonia cases. This assumed relationship is likely to be imprecise because certain agents, like respiratory syncytial virus, are more virulent than others, such as parainfluenza virus type 1.

The proportion of lower respiratory tract illness/pneumonia cases attributed to a particular pathogen sometimes differed between studies. In these instances, intermediate values have been used in the calculations if reported figures vary considerably. The resulting distribution of deaths due to noninfluenza ARI is assumed to be as follows: RSV, 7 percent; parainfluenza viruses, 5.5 percent; H. influenzae, 11.5 percent; and S. pneumoniae, 22.5 percent.

Table B.4 shows the results of combining the above assumptions with the estimates of annual noninfluenza ARI mortality.

To complete the disease burden estimates in the format required for the disease comparison method used in this report, it is necessary to estimate the number of disease episodes at various levels of severity. No specific information on the ratio of deaths to severe cases of ARI is available. However, the number of severe cases of parainfluenza and RSV disease can be calculated by presuming a case fatality rate of 10 percent for severe cases of these diseases. The relative distributions of less severe episodes are assumed to be the same as those estimated

TABLE B.5 Relative Case Frequenciesa

Category

H. influenzae

Parainfluenza Viruses

Respiratory Syncytial Virus

S. pneumoniae

Mild (A)

 

500

300

 

Moderate (B)

 

100

100

 

Severe (C)

7

10

10

7

Death (H)

1

1

1

1

aThese ratios are assumed from limited data (see text).



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