Basic Reproductive Rate (Ro)

Definition: The average number of persons infected by a single disease source. In other words, this is the number of expected secondary infections resulting from a single infectious case. Another way to explain Ro is the number of people who are expected to be infected by one person who has the disease in question.

This rate is affected by several factors including:

In general, if Ro is greater than one, the disease will continue to spread within a population. If Ro is less than one, the disease will eventually disappear from a population.

Ro is calculated using the formula Ro = C * P * D

Example: This chart displays Ro, the number of expected secondary transmissions from a single case of disease, for several vaccine preventable diseases. This chart also displays the estimated herd immunity thresholds needed to stop transmission for several communicable diseases. Notice that smallpox is actually less "transmissible" than measles and pertussis. However, even if desired herd immunity levels are reached, outbreaks of the disease can and still do occur, though not to the extent they occurred before larger-scale immunity was achieved.

 

Immunization Levels

Disease

Ro (# of usual secondary transmissions)

Herd Immunity

1999
19-35 Months

1997-1998
Pre-School

Diphtheria

6-7

85%*

83%*

9%

Measles

12-18

83-94%

92%

96%

Mumps

4-7

75-86%

92%

97%

Pertussis

12-17

92-94%

83%*

97%

Polio

5-7

80-86%

90%

97%

Rubella

6-7

83-85%

92%

97%

Smallpox

5-7

80-85%

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* 4 doses

Modified from Epid Rev 1993; 15: 265-302, Am J Prev Med 2001; 20 (4S): 88-153, MMWR 2000; 49 (SS-9); 27-38

 

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