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:

• The duration of infectivity
• The infectiousness of the organism
• The number of susceptible people with whom the infected patient comes in contact

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

• C = the number of contacts the infectious person makes per unit time (day, week, month, etc.)
• P = the probability of transmission per contact with the infectious person
• D = the duration that the infected person is infectious to others

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% __ __

* 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