For anyone interested:
Hope-Simpson pointed out several anomalies related to the seasonality of influenza. He proposed that the simple model of a continual process of "sick to well" transmission of the virus in a continuing chain can't explain these inconsistencies. He believed that there was some unappreciated or unknown aspect to the Sun/Solar radiation/Seasonal variation that was key to understanding this.
This article written by some Vitamin D proponents in 2008 provides a review of his ideas and speculates that Vitamin D might answer a lot of the questions Hope-Simpson raised. Later work seems to be a little mixed as to the efficacy of Vitamin D in relation to Influenza and the Flu, however these Influenza anomalies do seem noteworthy.
On the epidemiology of influenzahttps://virologyj.biomedcentral.com/articles/10.1186/1743-422X-5-29They summarize Simpson's "7 conundrums" as
1. Why is influenza both seasonal and ubiquitous and where is the virus between epidemics?2. Why are the epidemics so explosive? 3. Why do epidemics end so abruptly?4. What explains the frequent coincidental timing of epidemics in countries of similar latitudes?5. Why is the serial interval obscure?6. Why is the secondary attack rate so low?7. Why did epidemics in previous ages spread so rapidly, despite the lack of modern transport?
and they add an 8th and 9th that may be germane to Covid discussions
An eighth conundrum one not addressed by Hope-Simpson is the surprising percentage of seronegative volunteers who either escape infection or develop only minor illness after being experimentally inoculated with a novel influenza virus......A ninth conundrum evident only recently is that epidemiological studies question vaccine effectiveness, contrary to randomized controlled trials, which show vaccines to be effective. For example, influenza mortality and hospitalization rates for older Americans significantly increased in the 80's and 90's, during the same time that influenza vaccination rates for elderly Americans dramatically increased [7, 8]. Even when aging of the population is accounted for, death rates of the most immunized age group did not decline [9]. Rizzo et al studying Italian elderly, concluded, "We found no evidence of reduction in influenza-related mortality in the last 15 years, despite the concomitant increase of influenza vaccination coverage from ~10% to ~60%" [10]. Given that influenza vaccinations increase adaptive immunity, why don't epidemiological studies show increasing vaccination rates are translating into decreasing illness? 
