Has influenza been almost completely wiped out?

Question

According to this article, Simon de Lusignan – professor of primary care at the University of Oxford and director of the Royal College of GPs research and surveillance centre said influenza has now been 'almost completely wiped out.'

In that article this has been partly attributed to factors like masks, hygiene, social distancing measures, immunity and restricted international travel.

Professor Martin Marshall, chairman of the Royal College of GPs said:

It makes sense when you consider the lockdown restrictions, social distancing measures, and increased focus on maintaining good hygiene practices we are seeing at the moment — which will work to stop the spread of contagious illnesses such as colds and flu, as well as Covid-19.

So, they say both influenza and coronavirus could be reduced by 'the measures'. But only influenza actually has been reduced; almost to the point where it's been eliminated completely?

If influenza has been almost completely eliminated by the measures, why have coronavirus cases risen during the same period? How can this be possible?

If coronavirus has increased, then it logically follows that influenza cannot have decreased, since the same measures are said to reduce both.

Answer 1

Have the number of cases of influenza been massively reduced? Yes.

The World Health Organization track the number of cases of influenza. Their 18 Jan 2021 update covers this.

While they warn that the changes in health seeking behaviours mean the numbers should be interpreted with caution, they say:

Globally, despite continued or even increased testing for influenza in some countries, influenza activity remained at lower levels than expected for this time of the year.

They then go through each region - Western Africa continues to have influenza activity, but other regions have lower than normal or remain at "interseasonal" levels.

Is the disease "almost completely wiped out"? No

If we interpret that to mean we are close to eradicating influenza like smallpox then no. There are still a lot of human cases in absolute terms. They are mainly Influenza Type B, but plenty of examples of Influenza Type A. That is significant, because Influenza Type A is a zoonotic virus (i.e. it infects animals too.) It remains in animal reservoirs and even if we were capable of wiping it out of human populations, briefly, we would still see it come back from the wild animal populations.

But how?

The question includes some conjecture that coronavirus levels can't grow while influenza drops. I am largely ignoring this; the empirical evidence simply shows it isn't true.

Answer 2

If coronavirus has increased, then it logically follows that influenza cannot have decreased, since the same measures reduce both.

The question hinges on this assumption. I am not an epidemiologist, and this question probably belongs on a medical forum, but I'll give it a go.

Influenza and COVID-19 transmission differ in major ways. How long one is infectious, how long one is asymptomatic, and how easily it is spread.

COVID-19 seems to spread more easily than flu and causes more serious illnesses in some people. It can also take longer before people show symptoms and people can be contagious for longer.

Source: CDC - COVID-19 Symptoms

COVID-19 is more contagious and contagious for longer. People with COVID-19 are more likely to be asymptomatic while contagious meaning they will be less able to avoid infecting people, and people will be less able to avoid infection.

More detail on that is below.

Diseases require a certain rate of transmission to grow. For example, if every 100 cases infect 150 people it will grow. If every 100 cases infect 75 people, it will shrink.

Let's say influenza infects at a rate of 200%; every case on average infects two people. And COVID, being more infectious, goes at a rate of 300%; every case on average infects three people. These numbers are made up, point is COVID is more infectious than influenza. At these rates both will grow.

Let's say something happens and those rates are cut in half. Influenza is at 100%, just able to sustain its population. COVID would be at 150%, still growing.

Influenza

From the CDC

The time from when a person is exposed and infected with flu to when symptoms begin is about 2 days, but can range from about 1 to 4 days.

People with flu are most contagious in the first 3-4 days after their illness begins. Some otherwise healthy adults may be able to infect others beginning 1 day before symptoms develop and up to 5 to 7 days after becoming sick.

COVID 19

From the CDC

The virus that causes COVID-19 appears to spread more efficiently than influenza but not as efficiently as measles, which is among the most contagious viruses known to affect people.

Infections occur mainly through exposure to respiratory droplets when a person is in close contact with someone who has COVID-19.

COVID-19 can sometimes be spread by airborne transmission

Symptoms may appear 2-14 days after exposure to the virus.

Answer 3

This phenomenon has been known for decades, and it is sometimes called "the vanishing trick":

Andrewes was deeply puzzled: " ... strange as it may seem these A primes seem to have completely replaced the classical As allover the world. How this comes about and why the classical As should have vanished is a mystery."13 A mystery indeed! The phenomenon, christened the vanishing trick, has characterized most subsequent major and minor antigenic changes of influenza A virus. Strains that have been causing all the type A influenza in the world for perhaps a dozen years will vanish and next season be replaced everywhere by a novel strain. In the case of minor antigenic changes the predecessor may have been prevalent for only one or two seasons over a large part of the earth's surface before it disappears and is replaced by a new minor variant.

The vanishing trick still remains to be explained. In 1975, Edwin Kilbourne wrote:

... no less remarkable than the sudden appearance of major antigenic variants of influenza A virus as a concomitant of pandemic disease is the seemingly simultaneous disappearance of the antecedent virus from natural circulation. This is all the more remarkable because of the retention and preservation of potentially infective influenza A subtypes of the past in virological and diagnostic laboratories.14

Robert Webster and Graeme Laver after a discussion of the problem in 1975 concluded that there is no satisfactory explanation of the phenomenon available.13 A phenomenon that seems to defy logical explanation is surely calling attention to some fundamental feature of the epidemic mechanism and poses a challenge that cannot be ignored by any valid concept of the epidemiology of type A influenza. It is not yet certain if the vanishing trick also characterizes the behavior of influenza B viruses.

– R. Edgar Hope-Simpson: "The Transmission of Epidemic Influenza, Springer, 1992"

A new influenza virus enters a population, and a flu virus that was previously endemic disappears. The reason behind it is not understood, but it does not appear to have anything to do with viral transmission mitigations. The fact that it is also happened with COVID might be nothing new.