How We Know Flu Really Did Disappear in 2020 and 2021

Flu largely disappeared during 2020 and 2021, giving rise to ideas among some sceptics that it was being missed and wrongly classified as Covid.

Recently, some Covid number-crunchers – Martin Neil, Norman Fenton, Jonathan Engler and Nick Hudson – have given this theory a fresh lease of life with articles on Substack and in TCW Defending Freedom.

Their central claim is that U.K. flu surveillance was failing to pick up signals of what was in reality a normal flu season with ordinary influenza prevalence:

Our investigations into flu surveillance use in the U.K. suggest they were either not picking up signals that were in fact present or were (intentionally?) deactivated for the duration of what would have been a normal flu season in 2020-2021.

Their articles examine a number of data sources. However, a key one missed out is the “Respiratory DataMart weekly positivity for influenza” found in the ‘Weekly national Influenza and COVID-19 surveillance report‘ from the UKHSA. The following chart is from August 2022 (p12).

The data in the chart are based on thousands of clinical samples sent each week by hospitals to labs for testing for a variety of common viruses (in week 33 of 2022, for instance, 33,059 respiratory specimens were reported through the Respiratory DataMart System). It can clearly be seen that influenza positivity from these samples was near zero throughout 2020 and 2021, before a return to moderate levels in spring 2022.

This shows that testing of clinical samples for influenza continued throughout the pandemic at a rate of thousands per week and that the proportion of these samples which came back positive remained close to zero throughout. This is compelling evidence that influenza was genuinely absent as there is no other plausible way to explain these near-100% proportions of negative test results in thousands of surveillance samples. If influenza was present at normal levels, why are all these thousands of samples each week coming back negative?

This allows us to be confident that influenza really did largely disappear during 2020-2021 and was not merely being missed or misclassified.

The most likely explanation for influenza’s disappearance is viral interference, whereby more infectious viruses outcompete less infectious strains. This is the process by which new Covid variants completely displace older variants in a matter of weeks, and it appears that it can also operate between viruses. Plague Chronicle author Eugyppius has spotlighted the science behind this process. He draws our attention to an article in Science that cites some recent research on the topic. From Science:

In one of the largest, longest, and most comprehensive studies of respiratory virus infections in humans, Murcia and colleagues used a PCR assay that can identify members of 11 viral families to probe nasal and throat samples from more than 36,000 individuals who sought care from the National Health Service in Glasgow over nine years. Among other examples of viral interference, their data clearly showed rhinovirus and influenza A peaked at different times… demonstrating a “negative interaction” between the two viruses, the group concluded in the December 26th 2019 issue of the Proceedings of the National Academy of Sciences.

The Science article also cites a 2020 Lancet study that found that out of 1,911 samples positive for rhinovirus or influenza A virus, only 12 had both viruses.

Eugyppius discusses the potential biological mechanisms behind the phenomenon, noting the remarkable fact that overall prevalence of respiratory illness is relatively stable over time even as individual viruses wax and wane.

The claimed interference mechanism, as always, is interferon. This is an antiviral protein released in response to virus infection, which limits the capacity of other infecting viruses to replicate for a refractory period that may last as long as a month.

I’m sure that interferon plays an important role, but it’s probably only one of multiple factors at work here. To begin with, there’s one of my favourite hardly acknowledged virus facts, namely that “the overall prevalence of any viral respiratory infection among patients with respiratory illness is relatively stable over time, despite strongly varying prevalences of individual viruses”. Relatedly, we know from fever gauge data that overall rates of respiratory illness appear to be capped at around 10% of the whole population, and that rates of infection tend to collapse directly after reaching this ceiling… While the rate of virus infections remains relatively constant, the mix of viruses is always changing.

The phenomenon of viral interference thus seems to supply a plausible explanation for the temporary disappearance of influenza during 2020 and 2021.

There are a couple of other problems with the Neil et al. article that are worth noting. In the summary on Substack the authors state that “The NOMIS system shows that for the whole of 2020 there was no significant change in the number of deaths in the U.K. caused by flu (20,130), compared to previous years”. However, lower down it becomes clear that these are “Influenza and pneumonia” deaths, not just flu deaths, so may include pneumonia deaths with COVID-19, meaning the figures do not necessarily denote the presence of influenza.

Secondly, on Substack the authors write that “In January 2021 the ONS reported that there were almost as many deaths involving flu (7,610) as there were involving COVID-19 (5,719)”. In TCW these figures are switched: “In January 2021 the ONS reported that there were almost as many deaths involving flu (5,719) as there were involving Covid-19 (7,610).” Either way, though, the ONS data supplied with the weekly deaths bulletins show at least 30,157 deaths with Covid in England and Wales in January 2021, so I am unclear on the source of the much lower figures in the articles.

In sum, there are solid reasons to believe that the influenza virus really did drop to negligible levels during 2020-2021. Weekly U.K. surveillance testing of thousands of clinical samples consistently found zero or negligible percentages coming back positive for influenza. There is no other plausible way to explain this than that influenza was absent from the samples, and thus from the clinical picture. The most likely explanation is that it was outcompeted by the more infectious coronavirus.