In several quarters there is concern about a resurgence of the SARS-CoV-2 virus after the lifting of lock-down requirements. There is also understandable concern that a full-blown, second wave of the virus may hit countries later in the year. The UK Prime Minister has spoken of the inevitability of a second wave: this could have greater or lesser impact than the current outbreak. It is worth reflecting, therefore, on the secondary waves that some previous pandemics have created.
In the Spanish flu pandemic in 1918-1920, there were three different waves resulting in 40-50 million deaths worldwide overall. The first wave lasted from spring to summer 1918 and was relatively mild: it resembled typical flu epidemics with those most at risk being the sick and elderly while younger, healthier people recovered relatively easily. There were no reported quarantines during spring 1918.
Reported cases of Spanish flu dropped off over the summer of 1918 and there was hope at the beginning of August that year that the virus had run its course. In retrospect, it was only the calm before the storm. The second wave which began in the second half of August and lasted until December was much more deadly than the first: October 1918 was the month with the highest fatality rate of the whole pandemic, with younger men worst affected.
The reason why there was a second wave of the flu is still debated but is usually associated with broader public-health policy failures and issues related to the war such as soldiers travelling across Europe. Some suggest that the virus may have mutated into a deadlier strain between the first and second wave but this remains uncertain.
A third wave of illness occurred during the winter and spring of 1919 and subsided during that summer. This wave erupted in Australia in January and eventually worked its way back to Europe and the USA. The mortality rate of the third wave was just as high as the second wave but the end of the First World War removed the conditions that allowed the disease to spread so far and so quickly. Global deaths from the third wave, while still in the millions, paled in comparison to the catastrophic losses of the second wave.
Overall, the 1918 pandemic disproportionately killed young adults while school-aged children and the elderly were relatively unaffected. Other influenza pandemics have also produced shifts in the age structure of infections, as compared with patterns observed for seasonal influenza, including a lessening of elderly infections.
The Asian flu pandemic in 1957-1958, although milder in nature, was still significant. It resulted in 1-2 million deaths worldwide, with a higher mortality in younger age groups. Like the 1918 Spanish flu, the Asian flu appeared in waves although this time only two occurred, with the second wave proving deadlier than the first. The first wave initially appeared in the spring and summer of 1957. The second wave occurred in the autumn of 1957 and appeared to coincide with the re-opening of schools and the subsequent broader, community-wide transmission.
Hong Kong Flu
One decade later, the next of the severe pandemics emerged with the Hong Kong flu in 1968-1970. A unique feature of this virus was its accelerated spread due to extensive air travel which had not been seen in previous pandemics. Once again, there was a shift in the age distribution of mortality toward younger age groups. The Hong Kong flu is estimated to have caused between 500,000 to 2 million deaths worldwide.
There were two waves associated with the virus. The first began in July 1968, driven in part by Vietnam War veterans returning to the USA. The first wave was from 1968 until the winter of 1969, and the second wave occurred in the late autumn and winter of 1969-1970. Most influenza-related deaths in the first wave occurred in the USA (70%) and Canada (54%) while in Europe and Asia 70% of the deaths due to the pandemic were in the second wave.
The lessons from these three major pandemics show that beyond local resurgences there is a real danger that second and third waves can arise from serious viral infections. We should be mindful of this in our planning.