Modelling COVID-19 outbreaks in USA with distinct testing, lockdown speed and fatigue rates

Each state in the USA exhibited a unique response to the COVID-19 outbreak, along with variable levels of testing, leading to different actual case burdens in the country. In this study, via per capita testing dependent ascertainment rates, along with case and death data, we fit a minimal epidemic model for each state. We estimate infection-level responsive lockdown/self-quarantine entry and exit rates (representing government and behavioural reaction), along with the true number of cases as of 31 May 2020. Ultimately, we provide error-corrected estimates for commonly used metrics such as infection fatality ratio and overall case ascertainment for all 55 states and territories considered, along with the USA in aggregate, in order to correlate outbreak severity with first wave intervention attributes and suggest potential management strategies for future outbreaks. We observe a theoretically predicted inverse proportionality relation between outbreak size and lockdown rate, with scale dependent on the underlying reproduction number and simulations suggesting a critical population quarantine 'half-life' of 30 days independent of other model parameters.

Declining Vibrio parahaemolyticus and Salmonella, Increasing Campylobacter and Persisting Norovirus Food Poisonings: Inference Derived from Food Poisoning Statistics of Japan

The annual number of outbreaks of Vibrio parahaemolyticus and Salmonella food poisoning and that of patients in Japan, from 2000 to 2018, decreased exponentially even though the size of the individual outbreaks (the number of patients per outbreak) tended to become larger. For food poisonings caused by Campylobacter, the annual number of outbreaks increased exponentially while outbreak size became smaller and the annual number of patients remained almost unchanged. For food poisoning caused by norovirus, both the number of outbreaks and that of patients remained high throughout. Over time, the geographical and seasonal distribution of food poisonings became narrower for Vibrio parahaemolyticus and Salmonella, while they became wider for Campylobacter and norovirus. Further analyses using the attack rate-patient number plots suggested that the number of the outbreaks was determined mainly by the levels of microbial contamination of foods before they were brought into the facilities for consumption.

Characteristics of an epidemic outbreak with a large initial infection size

A deterministic model proposed in previous literatures to approximate the well-known Richards model is investigated. However, the model assumption of small initial value for infection size is released in the current manuscript. Taking the advantage of the closed form of solutions, we establish the epidemic characteristics of disease transmission: the outbreak size, the peak size and the turning point for the cumulative infected cases. It is shown that the usual disease outbreak threshold condition (the basic reproduction number R0 is greater than unity) fails to fully guarantee the existence of peaking time and turning point when the initial infection size is not relatively small. The epidemic characteristics not only depend on R0 but also on another index, the net reproduction number R0*.