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As the epidemiological situation intensifies, it is very important to discuss the relationship between seasonal respiratory influenza illnesses and the current epidemic. The symptoms of influenza illness largely overlap with the symptoms of SARS-CoV-2, so there may be a huge additional burden of diagnostic care in the coming months.
Reducing this is particularly important, so the widespread use of flu vaccination this year is also justified by scientific facts. The seasonal flu virus can infect up to 20 percent of the population each year and is responsible for an average of 650,000 deaths worldwide each year, of which 70,000 die in the European region. There are several questions related to this, which are worth clarifying: write in your abstract published on the mta.hu website two virologists from Pécs, Ferenc Jakab and Gábor Kemenesi.
How are seasonal flu and the SARS-CoV-2 virus alike?
Although from a virological point of view we are talking about members of two different virus families, from a functional point of view a series of similarities can be mentioned that place defense on a certain level.
Both viruses are transmitted by droplet infection through close contact and contaminated surfaces. Thus, the well-known norms of respiratory hygiene (use of a mask, distance, hand hygiene), which have been widely introduced in connection with the coronavirus epidemic, are adequate epidemiological measures for both diseases. The flu season is decreasing in the southern and northern hemispheres of the planet, following the seasons, so it officially ends just south of the Equator in August. It is interesting to find data on the cases there, it is true for the entire region that the measures introduced due to the Covid-19 disease resulted in orders of magnitude fewer cases of influenza. However, it is not known what to expect in the Northern Hemisphere throughout the current changed epidemiological strategies.
Both diseases can cause a very similar set of symptoms, ranging from asymptomatic manifestation to the appearance of moderate symptoms and a severe course such as a respiratory infection. Unfortunately, the general respiratory symptoms are the same in both cases, so a severe strain on the diagnostic network is expected.
How are the two viruses different?
One of the basic measures of epidemiology is the reproduction rate (R0), which, very simply, indicates the potential for the spread of pathogens. This indicates the growth rate of the epidemics, and if the value is less than 1 or 1, the epidemic is slowing down or stagnating. Basically, for Covid-19, this value is between 2 and 2.5, while for seasonal flu it is much lower. Of course, it is difficult to make a direct comparison, since the change in R0 is linked to several factors in each case. Currently, however, for Covid-19, as a new pathogen not yet known to the population, R0 is persistent and dynamically higher.
Although symptoms largely overlap, a more marked difference can already be seen in the proportion of severe cases. For Covid-19, 80 percent of infections are mild or asymptomatic, while unfortunately, 15 percent are severe manifestations that require hospitalization and an additional 5 percent require mechanical ventilation. This is an order of magnitude higher than that of seasonal influenza, and in terms of age group distribution, the hospital admission rate also affects the working age group to a greater extent in the case of Covid-19 today.
Children, the elderly, and pregnant women are the most affected by serious illnesses caused by the influenza virus. As far as we know, this is the case for Covid-19 in the elderly and those with a chronic or latent disease.
The most serious difference is in the death rate. Although the actual death rate from Covid-19 diseases awaits accurate cognition in the period to come and can vary greatly in space and time, available data and scientific knowledge suggest that it is at least ten times that of the seasonal influenza.
Unfortunately, the practical evidence for this is provided by the fact that, as of September 21, 960,000 people had died from infection caused by the new coronavirus, making us well above the average flu season at the threshold of the second wave.
The most important practical difference is that, while there are prevention (vaccine) and therapeutic (antiviral) options that work well for influenza, currently none are available for Covid-19.
What is expected?
Due to the measures introduced in relation to Covid-19, the southern influenza season has practically lagged behind since the existence of influenza surveillance systems. Therefore, what is expected at the epidemiological and disease level in the case of co-circulation of the novel coronavirus and influenza remains questionable for science.
The lag seems like good news, but as a result of the more lax management of the epidemic, what is expected in our region is not yet predictable. Unfortunately, if epidemiological measures are not properly maintained and applied, influenza could gain more ground in addition to the new coronavirus.
Extremely limited data are available on coinfections; So far, no differences in the course of other respiratory viruses have been observed in a small number of Covid-19 cases, in which other respiratory viruses have been examined.
Unfortunately, the lost flu season is disadvantageous in several ways. On the one hand, it carries the risk of a genetic bottleneck effect in the sense that among the low-circulation influenza strains, the most evolutionarily successful ones are subject to greater selection pressure, so that the most frequent strains may predominate. problematic.
A more practical problem is that the composition of influenza vaccines is determined on the basis of the most commonly observed strains year after year through a highly complex global monitoring system. Due to low circulation, we have much less reliable information on this this time for next season.
What can we do?
The effects of influenza vaccination on the course of Covid-19 disease have not yet been fully understood by science. In Brazil, a study involving more than 90,000 people is being carried out, which has not yet been the subject of scientific criticism, but the data is encouraging. Within the study population, a reduction in mortality was observed in Covid-19 patients who underwent influenza vaccination. The background for this is not yet known.
In no way can it be said that influenza vaccination provides protection against the new coronavirus, but it may have helped the outcome of the disease with a non-specific increase in immune capacity. More reassuring is that we have no known cases of conflicts of interest in Brazil or any other country that prevented the use of the influenza vaccine during the Covid-19 pandemic.
Regardless of the many unknowns, it is more important than ever to administer the flu vaccine in the current situation, as recommended by the World Health Organization (WHO) and the world’s leading professional organizations. In fact, the flu vaccine is not effective against Covid-19, but now the main consideration is to ease the network of diagnosis and medical care.
In addition to vaccination, it is very important to follow epidemiological measures: wear a mask, keep your distance, and wash and disinfect your hands regularly, regularly and thoroughly. With this, we can even “hit two birds with one stone” since we can avoid two viral infections of the respiratory tract.
Opening photo: MTI / Péter Komka
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