Influenza epidemics in temperate regions display dynamics that are characterized by pronounced seasonal peaks during the winter. The general lack of influenza cases during the off-season may result from the virus physically disappearing at the end of the season, in which case it must be imported annually. Alternatively, it may result from persistent asymptomatic carriers or unnoticed local transmission chains that develop into local epidemics as conditions become conducive. Here I attempt to understand these differing explanations by analyzing the global distribution of the four major subtypes that comprise influenza over a period of 18 years based on FluNet data, the surveillance network and database compiled by the WHO, and the NCBI influenza data resource, a repository of relevant genetic information. Examining the annual proportion of each subtype, I find considerable variations in subtype annual proportions between the regions. Moreover, I find that seasonal influenza subtypes can remain confined to specific temperate regions, without showing measurable global presence. These results indicate that although largely undetected during the off-season, influenza is likely to persist locally, and imply a ‘local-global’ model where annual influenza epidemics are a mixture of local strains undergoing reactivation together with an influx of global variants.

Since identified as the pathogen responsible for an outbreak of severe respiratory distress in Wuhan, China, the 2019-nCoV coronavirus has caused over 30M cases and 1M deaths globally. Sporadic cases were identified in several states in the US from early January, and large-scale community transmission is believed to have started in late February, leading to a first spike in COVID-19 deaths and overall mortality in late April, and a second spike later in the summer. I show here that the dynamics of the pandemic were different in different regions of the US, showing a north-south pattern, with a first pandemic wave mainly in northern regions, followed by a second wave mainly in southern regions. Analysis of overall mortality data shows that the increase in mortality correlates well with COVID-19 incidence in most regions, and that from April through August COVID-19 deaths accounted for a substantial proportion of all deaths in all parts of the US.

late in December 2019 2019-nCoV was identified as the pathogen responsible for an outbreak of severe respiratory distress in Wuhan, China. The virus was detected in multiple countries during January, but it is believed widespread community transmission began late in February or early March. Since March the virus has caused over 100k confirmed deaths in the US, with some states more severely impacted, notably NY and NJ. Here I examine excess mortality at the national and state level from January through July 2020. I find that the increase in excess mortality began in late February, suggesting the pathogen was circulating undetected earlier than assumed. The timing and intensity of the increase in excess mortality varied across states, with two patterns emerging: an early, sharp increase reaching a peak during April-May, best exemplified by NY and NJ, and a shallower, sustained increase, reaching a peak in late July, observed mostly in the southern regions of the US.

Seroprevalence studies suggest that the number of PCR-confirmed COVID-19 cases is significantly smaller than the true number of infections. I study logintidual seroprevalence data from 7 sites across the US, from early April 2020 to June 27. I show that not only COVID-19 seroprevalence does not seem to increase over time, there is no clear association between the number of cases reported during a period and the change in seroprevalence during the same time. I conclude that as they are, seroprevalence studies can only be used in the qualitative sense and distinguish between populations with no COVID-19 exposure, to those populations where the virus had already started spreading.

Despite great advances in understanding the dynamics of viral epidemics, the emergence of rapidly spreading, highly pathogenic viruses remains a realistic and catastrophic possibility, which current health systems may not be able to fully contain. An intriguing feature in many recent zoonotic viral outbreaks is the presence of superspreaders, which are infected individuals that cause dramatically more new cases than the average. Here I study the effect of superspreaders on the early dynamics of emerging viruses that have not gained the capacity for efficient human-to-human transmission, i.e viruses with R0 < 1. I show that superspreaders have a higher chance of rapid extinction, but under crowded conditions can lead to outbreaks, causing far more cases than regular viruses. I suggest that outbreaks of highly pathogenic superspreaders are more likely when they coincide in time and space with an unrelated outbreak leading to increased hospital admission rates. These superspreader outbreaks may be difficult to detect, especially in the context of a different epidemic in progress, and can significantly affect mortality patterns observed in affected areas.