Fearing the simultaneous circulation of coronavirus and flu viruses, in the second half of 2020 the Ministry of Health had recommended that the Regions anticipate the vaccination campaign against influenza, especially to protect people at risk such as the elderly and individuals with other health problems. . The greater number of vaccinated people, the strong limitations to reduce the circulation of the coronavirus and other variables had avoided the worst, making the flu season practically non-existent in the cold months between 2020 and 2021.
Something similar had happened in several other countries, which are now wondering what the new flu season that has just begun will be like in a still different context, where many limitations have been relaxed thanks to the protection offered by the coronavirus vaccines, which in the fall last year were not yet available.
Influence and seasonality
We tend to see the flu as a harmless seasonal ailment, an annoying disease that resolves itself after a few days and without leaving any consequences. In fact, flu syndromes are among the leading causes of death worldwide.
Depending on the years and the flu viruses in circulation, influenza causes between 4 and 50 million cases with symptoms in Europe, and it is estimated that it leads to the death of 15,000 to 70,000 individuals. The disease is more frequently associated with severe symptoms among children, the elderly and people with other health problems: thousands of individuals are hospitalized every year for flu and many of them are unable to survive the infection, due to the complications that can take over.
Influenza vaccines are an important tool for reducing risks, but the protection they offer varies greatly from year to year, because flu viruses have the ability to mutate very quickly and it is not always easy to predict which variants will circulate the most in the population in each flu season.
As in many other countries, in Italy, vaccination against influenza is recommended for most of the population, and it is completely free for people over 65 (the threshold has been reduced to also include the 60-64 age group. years during the pandemic).
The Higher Institute of Health (ISS) monitors the flu seasons through “InfluNet”, an integrated surveillance system that collects reports on cases found in Italy and which periodically produces reports on their progress. The data is provided by over a thousand “sentinel” doctors, which indicate how many of their patients have shown flu-like syndromes (ILI), and by 22 laboratories, which analyze samples taken from patients and sent by sentinel doctors to ascertain any presence of influenza viruses.
ILI usually have a regular trend: cases increase in the last weeks of the year and reach a peak at the end of January of the following year, which is maintained until March. The incidence can vary significantly from one year to the next, depending on how aggressive the influenza viruses are, the effectiveness of the vaccines and the vaccination coverage achieved.
Total incidence and age groups of ILIs from the 2004-2005 season to the 2020-2021 season (IfnfluNet)
The last flu season had a very low incidence of ILI, below the threshold of 3.16 cases per 1,000 patients. The figure is so low that it can be said that between autumn and winter the actual epidemic period of the flu never started.
The difference between the end of January 2021 and the end of January 2020 shows it effectively: this year the incidence of ILI was 1.4 cases per thousand assisted, while in the same month of the previous year it was 12.6 cases per thousand assisted. . The amount of ILI also remained very low even among children, usually the age group with the highest incidence.
It is difficult to establish with certainty the factors that determine a flu season that is more severe than another, especially if other variables such as a pandemic are involved, but some elements can still be identified.
In the autumn of 2020, the early start of the flu vaccination campaign had contributed to the achievement of 23.7 percent of Italians vaccinated, seven percentage points more than the previous year. Coverage in the elderly population over 64 had also reached 65.3 per cent, much higher than 54.6 per cent in 2019. The increase also affected the pediatric age groups, where usually no achieves significant coverage.
A greater share of vaccinated people has probably influenced the trend of influenza cases, even if, as we have seen, influenza vaccines have limited effectiveness in a few years.
Influenza viruses do not confer an immune memory for life, but studies conducted in recent decades have nonetheless revealed the presence of some immunity in the body, which is partly renewed with each new infection. This is not enough to prevent an infection, but it appears to help reduce the severity of symptoms.
In some years, the flu viruses in circulation have more in common with those of the previous season, and can therefore find it more difficult to spread due to the immune defenses already developed (although these have reduced over the course of about a year). The vaccine confers immunity without the risks of a full-fledged flu infection, and can help reduce virus cases and circulation, in the years when predictions have led to the development of the most suitable vaccines.
Influenza viruses have to compete every year with many other respiratory-borne viruses that cause flu-like diseases. They usually manage to win the competition in early winter, becoming predominant over other viruses. The strong circulation of the coronavirus in the cold season 2020-2021 could have contributed to the lower spread of influenza viruses, and the consequent lower incidence of ILI among the population.
Limitations and COVID-19
However, these elements are not sufficient to explain the substantial disappearance of flu viruses in the last flu season. According to various analyzes, the strong restrictions in place against the coronavirus last winter contributed to the reduction. The closure of numerous meeting places, the request to wear masks and physical distancing have considerably reduced the risks of contagion from influenza, the transmission of which takes place by air in a manner similar to those of COVID-19.
For much of the last flu season, the vaccines against the coronavirus were not yet available, the administration of which was massively started only towards the end of winter. Limitations were inevitable to keep infections under control and reduce the risk of having hospitals close to collapse due to the high number of COVID-19 hospitalizations, as had happened during the first wave in 2020.
Compared to the tail of last flu season, now the situation has changed significantly. Coronavirus vaccines have been given to billions of people, with a large reduction in severe cases of COVID-19 requiring hospitalization or causing the death of those who fall ill. Thanks to vaccinations, many countries, including Italy, have been able to remove most of the limitations that had entailed many sacrifices, in economic terms, of opportunities and with considerable consequences for the mental health of many.
In this new scenario, it is conceivable that with the arrival of the cold season the restrictions introduced last year will not be adopted again. For this reason, some researchers and analysts have reported the risk that other viruses, such as the flu, may return to circulate more easily among the population, with the risk of having a flu season with a high number of infections and sick people.
The risks related to the coexistence of coronavirus and flu viruses in the cold season could be higher in countries where there is no obligation to wear a mask indoors, as in various parts of the United States where there is also a high incidence of new positive coronavirus cases. In Italy, the obligation to wear masks still applies to most of the indoor environments and means of transport, while it does not apply to restaurants and some places (such as swimming pools and gyms) where the Green Pass is still required. In these circumstances, the circulation of influenza viruses should be contained, but still more significant than last year.
Although epidemiologists and virologists use various models, it is not easy to predict months in advance of the course of the flu and the spread of types of flu viruses. An analysis of those circulating in the other hemisphere can provide some clues as to what to expect, as well as a study of immunization levels among the population following the previous flu season.
Since practically no one got sick with the flu in the cold months of the 2020-2021 period, at this moment there are very few people with an immunity (however partial, for the reasons we had seen before) that can protect them at least from the most severe symptoms. severe infections caused by flu viruses. This is probably especially true for the groups of the population most at risk, starting with the elderly who have lived in greater isolation in the last year and a half compared to the younger ones who have continued to work and go to school, albeit intermittently in depending on the limitations in force in the last year.
Experts therefore report that this flu season will be even more important than usual to get vaccinated against the flu.
In Italy, the Ministry of Health has recommended to the regions also for the 2021-2022 flu season to provide free vaccines in the age group between 60 and 64 years, in addition to the free administration already provided for those over 65. Vaccination is also strongly recommended by the ministry for “health and socio-health professions who work in contact with patients, and the elderly institutionalized in residential or long-term care facilities” and for the “6 months – 6 age group. years, also in order to reduce the circulation of the influenza virus between adults and the elderly ».