The prevention measures implemented by the countries have meant that influenza infections have hardly been detected in the world. However, the cases of covid-19 do not stop increasing and already exceed the sad figure of 100 million infected and 2 million deaths.
How is this possible if masks and protective measures should work for both viruses? Are we facing a worldwide conspiracy to make us believe that the cases of covid-19 are increasing when it really corresponds to the annual peak of flu?
It is normal for this apparent contradiction to generate confusion. This is what happens when you try to compare two viruses that, although similar in some ways, are very different.
Below we can find some of the reasons why, although prevention measures work for both viruses, they do not affect both equally:
1. The flu virus is seasonal
The flu virus is a seasonal virus whose incidence is highest in the winter months. In other words, it is a virus that continuously circulates around the world and jumps from one hemisphere to another according to the corresponding season. In general, from December to March there are more cases in the northern hemisphere; from June to September, in the southern hemisphere.
Therefore, the restriction of mobility between countries and the decrease in the number of trips that we have seen during the pandemic have been able to favor a decrease in the spread of the influenza virus from one hemisphere to another.
However, contrary to what happens with the flu virus, at the moment the virus responsible for covid-19 is not a seasonal virus. Consequently, it would not be so affected by mobility restrictions between countries.
2. This year more people have been vaccinated against flu
The flu vaccination campaign has been advanced in 2020 with special attention to nursing homes and health care to avoid overlapping with the coronavirus.
In addition, this year more people have been vaccinated than in previous years, so there would be more people immune to the virus.
3. The incubation periods are different
The incubation period of the coronavirus is between 2 and 14 (or even more) days, a much longer period than that of the flu, which is usually 1 to 4 days. As a consequence, if a person has COVID-19, they could infect people for a longer period of time than if they had the flu.
4. The coronavirus is more contagious
Although the covid-19 virus and the flu virus are respiratory viruses that spread in a similar way, the coronavirus is more contagious among certain populations and age groups than the flu.
In general, it is estimated that the number of people who would be infected from an infected individual (basic reproductive number or R₀) is 1.3 in the case of the flu and 2 to 2.5 in the case of the coronavirus .
5. Transmission by aerosols is essential
Studies seem to indicate that aerosol transmission could play a very important role in the transmission of the coronavirus. Aerosols are particles that are produced when humans breathe, speak, sing or laugh (even in asymptomatic individuals) and remain suspended in the air for longer than other larger droplets. The main problem with aerosols is that they are so small that they can remain in the air, reach more than 2 meters and accumulate in high concentrations in poorly ventilated places.
On the other hand, it is considered that the main source of transmission of the influenza virus is through larger drops, which do not reach very far (about 2 meters) and fall to the ground relatively faster as they are heavier.
Therefore, to become infected with the influenza virus, it would be necessary to be quite close to the infected individual, which is largely avoided thanks to the safety distance that so many countries have implemented.
6. The coronavirus generates more super-spreaders
The coronavirus has been observed to generate more cases of super-spreaders than the influenza virus. A super-spreader is a patient who can transmit an infection to a large number of people compared to the average patient. In general, super-spreads are events that have a very important significant impact on how a disease spreads.
This means that the coronavirus can spread quickly and easily to many, many people and cause continuous spread between people as time progresses.
7. The asymptomatic
The coronavirus infection generates a large number of individuals without symptoms who, not knowing that they are infected by the virus, continue to lead a normal life, distributing the virus to other people – even if they later develop them. In fact, the latest studies indicate that individuals without symptoms would be responsible for more than half of the coronavirus infections.
It is true that infection by the influenza virus also produces asymptomatic infections, but the main difference is that in these cases it usually occurs due to the existence of a previous partial immunity to the virus. Therefore, although these individuals are infected and can infect, they do not do so with the same intensity as those with symptoms.
8. Coronavirus infection could prevent other infections
Coronavirus infection could prevent infection by other viruses in viral interference phenomenon. Very briefly, in some cases, infection with a virus makes it more difficult for that person to be infected with another virus for a certain time. This usually happens because the first infection creates an “antiviral state” that prevents other infections.
In summary, although the pandemic may have affected the diagnostic capacity of flu cases, the decline in flu cases is real and has a logical explanation. Therefore, attributing the increase in covid-19 cases to undiagnosed flu or thinking that masks allow some viruses to pass through to others is not too simplistic and does not reflect the real complexity of the situation.
Jose M Jimenez Guardeño, Researcher in the Department of Infectious Diseases, King’s College London and Ana María Ortega-Prieto, Postdoctoral research associate, King’s College London
This article was originally published on The Conversation. Read the original.
