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Widespread Pandemic Antibody Confounds Clinical Trial
Recombinomics Commentary 16:02
September 22, 2009

At baseline, 76 of 240 subjects (31.7%) had antibody titers of 1:40 or more on hemagglutination-inhibition assay

The above comments are from the recently completed clinical trial for H1N1 in healthy adults in Australia.  Although cases with confirmed pandemic H1N1 were excluded from the study, over 31% of the participants had protective antibody prior to the first infection, signaling widespread influenza in undiagnosed adults.  This high level is likely linked to limited testing, as well as the use of fever in the case definition.  Thus, those without fever are not tested and are significantly under-represented in confirmed cases.

Data on symptomatic patients without fever suggests the frequency of infection without fever is above 50%.  30% of initial lab confirmed patients in Mexico, who were hospitalized because of breathing difficulties, did not have a fever.  In Chile, the frequency of confirmed patients without fever was 50%.  A recent study of transmission during a plane flight found that 36% of lab confirmed cases did not have a fever.

However, these figures are likely to be higher in the general population, because mild cases with no fever will not seek medical attention, or will not be tested because they do not meet the case definition, which includes fever.

Similarly, students without fever will be largely missed because guidelines cite fever AND a subset of additional symptoms, so those without fever are not considered to be infected patients.  Thus, they are misdiagnosed as having colds, allergies, strep throat, bronchitis, or stomach flu, and such students frequently remain in school, infecting other students (see map).

This leads to a high percentage of infected cases in clusters that are not tested in general, and those that are tested are almost exclusively cases with fever.

Thus, the inclusion of fever in the case definition ensures significant spread by patients without fever as well as a major undercount of infections.  This selective testing limits surveillance, and creates a large reservoir of virus in patients who have not been properly diagnosed.

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