22-06-21 11:00:00, Summer wave driven by the ‘Indian variant’ (delta) in the UK, Russia and Portugal (OWD)
On the ‘Indian variant’ summer wave and the mysteries of respiratory virus seasonality.
Since April/May 2020, when coronavirus infections temporarily vanished even in countries with almost no “interventions” (such as Sweden), it has been quite clear that transmission of the novel coronavirus is primarily determined by seasonal influences, similar to influenza viruses and other human coronaviruses (see charts below).
This assessment has later been confirmed by the stunning synchronicity of coronavirus infection rates in neighboring US and European states with entirely different “covid policies” (e.g. concerning face masks, school closures and business closures). Thus, most health authorities have dramatically overestimated the impact of their coronavirus policies (see charts below).
Interestingly, however, the new ‘Indian variant’ of the coronavirus (delta) appears to be the first major exception to this seasonal pattern (1). In the three northern hemisphere countries in which it has already established itself (and only in these!) – the UK, Portugal and Russia – infections in June have markedly increased (see chart above).
In the highly-vaccinated UK, the impact on hospitalizations appears to be limited so far, but Portugal and Russia – especially Moscow – report a significant strain on their health care systems and have already re-imposed new (partial) lockdowns (Russia has already lost 500,000 people to covid). Moreover, China and even Israel are also reporting new outbreaks driven by the Indian variant.
Respiratory virus transmission and seasonality
Is is important to note that while the seasonality of many respiratory viruses is well established (see charts below), virologists and epidemiologists still do not understand the factors driving this phenomenon, which is why their models fared so poorly during the covid pandemic.
Animal studies have shown that both temperature and relative humidity influence aerosol transmission, essentially bringing infections to a stop at high temperatures or high relative humidity (2). But this doesn’t explain respiratory virus transmission dynamics in the tropical and sub-tropical regions, which have never followed a typical seasonal pattern anyway (see charts below).