According to the WHO, the lethality of COVID-19 is higher than for seasonal influenza (usually well below 0.1%). COVID-19 mortality seems to differ according to the region.6 Until now, it is estimated that the crude mortality ratio (the number of reported deaths divided by the reported cases) is between 3-4%; therefore, the infection mortality rate (the number of reported deaths divided by the number of infections) will be lower. However, for both diseases, mortality is determined to a large extent by regional access to and quality of healthcare services.
In our country, the functional healthcare and surveillance systems installed, the government’s efforts to identify ILI cases during the COVID-19 pandemic, and adequate laboratory capacity guarantee an appropriate influenza testing and reporting of results. As reported by other countries, public health initiatives to control COVID-19 spread probably helped to decrease influenza transmission and number of cases in May 2020 because both viruses share the same transmission mechanisms through the respiratory route and contact, although with different efficiency giving a basic reproduction number (R0) for COVID-19 higher than that of seasonal influenza, then a reduction in transmission could reduce the impact of COVID-19, thereby preventing the increase of mortality. The modeling of the effective reproduction number for COVID-19 in Mexico in May 2020 (National Secretariat of Health) at 0.5–1, is lower (55-77% less) than the mean estimated R0 for this virus (2.2).7 The observed 61% reduction in influenza transmission is consistent with the information above.8
Due to the measures implemented in our country to contain the COVID-19 cases, there were severe changes in the availability of medical care and assistance, including hospitalization. Avoidance of medical care during this period may be a major confounder in interpreting our results. It is important to note that due to the similarity in symptoms between COVID-19 and influenza and the low number of COVID-19 patients in Mexico (<200 cases as of March 21, 2020), ILI patients would seek help for a differential diagnosis. The reduction of the availability of medical care also did not explain the lower number of severe influenza cases seen in 2020 (Figure 2C). Therefore, we believe that the decline in influenza activity in Mexico in 2020 is the result of the strict control measures that were put in place in response to COVID-19.
During the last winter season, European countries and the North American region (United States, Canada and Northern Mexico) have reported fluctuations in the number of cases of seasonal influenza different from experienced in previous seasons (since EPI week 4, 2019–20 season) European countries and North America region (United States, Canada and North Mexico) have reported similar trends in the fluctuation of seasonal influenza cases from those experienced in previous seasons. In contrast, some Asian regions experienced different trends in influenza incidence during this season, from those experienced in past years.9,10 The total number of patients infected with influenza in countries like Hong Kong, Japan, and Taiwan was significantly lower after EPI week 4 of the 2019–20 season than in previous influenza seasons.11
We can mention some limitations in our study. First, due to the seasonal bimodal pattern of influenza incidence in Mexico, it is expected that influenza cases diminish around March-May. However, this year the number of cases fell to almost zero. Second, it is important to consider an underdiagnosis of ILI´s due to the fear of leaving home in search of medical attention and catching coronavirus, as well as being classified as COVID-19, all this contributes to generating an altered pattern11 Third, if we consider a similar dynamic for COVID-19, we will also have to record a higher number of cases for COVID-19 due to the underdiagnosis and the presentation of asymptomatic cases.12,13
In conclusion, we found a marked decline in influenza cases in Mexico after the implementation of public health measures for COVID-19. Definitively, the containment measures to reduce the risk of contagion for the new respiratory disease was additionally efficient for influenza, but with greater magnitude. This is partly explained by the different transmission mechanisms of these two respiratory diseases, the virulence of each viral agent, the absence of immunity in the population, and the diverse virological factors inherent in each pathogen. Our results suggest that such measures are useful in reducing the spread of viral respiratory diseases and their establishment mitigate the impact of the COVID-19 pandemic, especially now that we are entering into a new influenza season.