Abstract: Objective To analyze the time series of incidence rate of scarlet fever in Nanjing from 1951 to 2020, and to understand its variation characteristics and trends. Methods The moving average, sliding t test and time-frequency method of wavelet transform were used to analyze the incidence rate of scarlet fever in Nanjing over the period of 1951-2020. Results Scarlet fever experience showed two peaks over the year in spring summer season and winter season respectively in Nanjing. The annual incidence rate was decreasing, with a decreasing rate of 0.144 8 （incidence rate /5 years）. The sliding t test showed 4 descending mutation points and 3 rising mutation points. The time-frequency analysis of Morlet wavelet method showed 3 central scales and cycles, which were 4, 16 and 25 scales respectively, and the high and low incidence cycles were 4, 8 and 13 years respectively. The first main cycle was on the scale of 25. From the cycle analysis results, the incidence of scarlet fever had been rising since 2014. At the beginning of 2020, during the first and second levels of public health emergency response, the incidence decreased significantly, and the incidence increased during the third level response. Conclusion According to the analysis of periodicity and trend of scarlet fever, we should strengthen the monitoring of scarlet fever and deal with the epidemic situation in a timely manner, so as to control its transmission within the population.

Key words: Scarlet fever, Sliding t test, Wavelet analysis, Periodicity