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Nonclassic measles infections in an immune population exposed to measles during a college bus trip

This study investigated the frequency of mild or asymptomatic measles infections among 44 persons exposed to a student with measles during a 3‐day bus trip using two buses. Questionnaires and serum samples were obtained 26–37 days after the trip. All participants had detectable measles‐neutralizing antibodies, and none developed classic measles symptoms. Ten persons (23%) were IgM positive for measles, indicating recent infection. Among previously vaccinated IgM‐negative persons, those who rode on bus A with the index case‐patient had significantly higher microneutralization titers than those on bus B (P = .001), suggesting that some persons on bus A were infected but were IgM negative at the time of the study. Mild or asymptomatic measles infections are probably very common among measles‐immune persons exposed to measles cases and may be the most common manifestation of measles during outbreaks in highly immune populations. J. Med. Virol. 56:337–341, 1998. © 1998 Wiley‐Liss, Inc.