Relation between Naturally Acquired Immunity and Infectivity of Two Rhinoviruses in Volunteers

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Abstract
Volunteers with various levels of preexisting serum antibody were challenged with two rhinoviruses for determination of infectivity curves. The 50% human infectious dose (HID50) for type 39 rhinovirus was 0.4, 2–3, and 6.5 TCID50 for subjects with titers of antibody before challenge of 50 for type 14 rhinovirus was 5.7 TCID50 for antibody-free volunteers and 33 TCID50 for subjects with levels of antibody of 2–32. The slopes of the infectivity curves for antibody-free volunteers were different for the two viruses; the slope for type 39 was steeper than that for type 14. Shedding of virus was a more sensitive indicator of infection than antibody response with either virus. Daily analysis of symptoms showed that the experimental infections produced illnesses that were similar to natural colds. An indirect estimate of the challenge dose of rhinoviruses under natural conditions was obtained by use of these infectivity curves and of the rates previously observed in natural infections due to rhinovirus. The estimated natural challenge doses ranged from 0.5 to 3.25 TCID50 for type 39 and approximated 80 TCID50 for type 14 virus.

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