Methods: The analysis begins with a search of historical medical reports on the relationship between varicella and scarlet
fever epidemics in the late 19th and early 20th century, as well as in more recent empirical studies. Correlations and crosscorrelations
between varicella and scarlet fever are analyzed using weekly public health case reports from 1924 to 1932
for Boston, Chicago, New York City, and Philadelphia. Regression models are used to estimate the relationship between
varicella and scarlet fever after controlling for seasonal forcing.
Results: Historical records give limited support for a causal relationship between varicella and scarlet fever but indicate that
these diseases often occurred close in time to each other. Likewise, statistical analysis shows that varicella and scarlet fever
epidemics are closely aligned with each other, and varicella has a strong relationship with scarlet fever. The relationship is
stronger than reported in any previous research on the two diseases.
Conclusion: The close correspondence of the two diseases likely depends on multiple factors, including seasonal forcing,
a causal relationship, and co-infections. The results raise questions about whether this close relationship might have had a
synergistic effect or a role in the evolution of S. pyogenes from the virulent, high incidence epidemics of the 19th century to
the relatively benign scarlet fever of the 1950s.