Flu pandemics recur reliably but unpredictably every decade or so, and their extent and intensity vary. With COVID-19, we may be in midst of a once-every-50-years event, perhaps similar to 1957 pandemic, but not as bad as the 1918 pandemic.
Let’s talk about the 1957 pandemic.
Pandemics result from the emergence of viral strains that are novel, often from genetic recombination in animal reservoirs. The 1957 pandemic was due to influenza A (H2N2). https://cdc.gov/flu/about/viruses/types.htm… “Serological archeology” suggests it resembled 1889 strain. https://ncbi.nlm.nih.gov/pubmed/8877331
The virus that caused the 1957 pandemic is different from the coronavirus causing COVID19. Both are rhinoviruses, but from different phyla. https://statnews.com/2020/03/03/who-coronavirus-different-than-influenza-can-be-contained/… Despite the different pathogens, we can still understand what is happening by studying past pandemics.
First, here is a timeline via the CDC of the history of public health responses to flu pandemics: https://www.cdc.gov/flu/pandemic-resources/pandemic-timeline-1930-and-beyond.htm
Best estimate at present is COVID-19 has intermediate transmissibility compared to other pandemics (an effective reproductive rate of 2-4 new cases per old case) and intermediate mortality (0.5%-2.0%, though still unclear).
A classic 1961 paper analyzing the 1957 pandemic https://ncbi.nlm.nih.gov/pubmed/13758900 concluded that the disease likely started in central China (like COVID-19) and became known to the rest of the world in April of 1957. Pandemics have started on many continents.
Globally, the 1957 pandemic killed 1.1M people. There was regional variation; for instance: 0.3 deaths/10,000 in Egypt and 9.8/10,000 in Chile. GDP and latitude explained 43% of the variance in excess mortality. https://t.co/vUZQfTez3X?amp=1
Mortality in the 1957 global flu pandemic was U-shaped, with the very young and very old dying (https://ncbi.nlm.nih.gov/pubmed/8877331). This is typical, but this appears NOT to be the case with COVID-19, where the young are spared.
The first wide awareness of the 1957 pandemic in the USA was a tiny article on April 17, 1957 in the New York Times on page 3 noting that 250,000 people were afflicted in Hong Kong (or 10% of the population). The 1957 pandemic was first recognized in USA in June in RI, but other outbreaks soon occurred in CA. By September, it was everywhere. And it recurred when schools re-opened in fall of 1957. A first peak in excess death was in October, and a second peak in February of 1958
Peaks in epidemics have to do with pathogen flows across networks; other social factors (like changes in population mixing across time, or school re-openings); weather; etc. As a note, the flu has a baseline seasonality.