There is much concern about a second, third or even fourth wave of Covid-19, implying an inevitable increase in the numbers infected. Proponents of this theory think variously that infections could reoccur immediately after lockdown, that there will be multiple waves over the ensuing years, or that there is worse to come next winter. The truth is, no one really knows.
The term “wave” as applied to infections originates from the 1889-92 influenza outbreak, supposed to have had two phases, the latter more severe. The current theories – used to inform most of the modelling studies – are based on accounts of the 1918-20 “Spanish flu”. This outbreak started in the “spring wave” and went on into the summer. The natural social distancing that occurred throughout summer led to a downturn in the infection rate before it picked up again in the winter. In several accounts, this later “wave” is described as being more severe.
There are several problems in using Spanish Flu to direct predictions about Covid-19. First, the actual number of deaths that occurred is highly uncertain. The Spanish flu virus had no genetic characteristics to explain its virulence, suggesting human factors were at play. One of the most powerful was the long and dreadful war, during which huge numbers of people congregated, moved, starved and died. The late summer of 1918 led to further mass movement, and by the end of the war, further concentration, transport and dispersal of people occurred.
Sequencing of influenza from RNA strands is only available for three affected individuals from the autumn of 1918. This evidence is used to infer that the flu caused many deaths. But the combination of war, movement and starvation means we cannot be sure that the same microorganism caused the preceding March-April outbreak.
The term “wave” suggests that infections are synchronous, visible, and solid. They are not. It also wrongly suggests that they all behave similarly.