The SARS-CoV-2 and influenza see-saw

Influenza spikes when Omicron leaves

Covid and influenza appear to have a reciprocal relationship. When a covid wave occurs influenza disappears and when covid recedes influenza returns. The fear of having to cope with covid and influenza at the same time over winter as warned in a report from Imperial this time last year, was not valid.

In spring 2020, influenza disappeared. Survey samples carried out at GP surgeries for people with influenza symptoms were no longer detecting it. It was a global phenomenon. Interestingly, influenza disappeared from Italy two weeks earlier than from Sweden and England and the covid surge coincided with the disappearance in each case.

Figure 1: WHO registered specimens of influenza for Italy, Sweden and England in 2019/20

Winter 2021/22 saw a return of influenza across the world but with an odd double peaked pattern (see figure 2). Pakistan was the first to see a return from the end of 2020 (see figure 3).

Figure 2: Total world influenza specimens registered with WHO since 2017

Figure 3: Influenza levels in Pakistan from 2017

More recent months have demonstrated a reciprocal relationship between influenza and SARS-CoV-2 with countries that have had a gap between Omicron waves seeing an influenza surge occur in the interval period, before it disappears again when Omicron returns. 

Other countries have seen an influenza wave interrupted by a covid wave. The United States illustrates this well. A clear bimodal influenza peak was seen recently which was not present in previous years (see figure 4). Superimposing the covid cases graph on the influenza chart shows that the fall in influenza coincides with the rise in covid cases (see figure 5). Sweden and Malaysia show a similar relationship (see figures 6 & 7).

Figure 4: Influenza specimens registered for the United States

Figure 5: Covid cases superimposed on recent influenza cases in United States

Figure 6: Covid cases superimposed on recent influenza cases in Sweden

Figure 7: Covid cases superimposed on recent influenza cases in Malaysia

For the last two years South Africa has had a large winter wave of covid. This year began as a covid wave but has been replaced by an influenza wave (see figure 8).

Figure 8:Covid cases superimposed on recent influenza cases in South Africa

In contrast, Israel which has also had a covid wave at this time of year for the past two years currently has a covid wave which has peaked with similar levels of cases and hospitalisations to this time last year.

Figure 9: Covid cases superimposed on recent influenza cases in Israel and Palestine

What does the return of influenza mean? Has South Africa seen the last of SARS-CoV-2 with a return to more usual winter respiratory pathogens? If so, why is Israel still experiencing significant covid waves? Is the competitive relationship only seen with Omicron and not other variants? What will it mean for our coming winter? Covid results in higher demand for intensive care capacity than influenza has done so the difference between a winter dominated by covid or by influenza could have an important impact on healthcare provision. 

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