This is the twenty-first document in a series of discussion notes addressing fundamental questions about the COVID-19 crisis and response strategies. These documents are based on the best scientific information available and may be updated as new information comes to light
Written by Anna Vilella (Hospital Clínic–ISGlobal), Gemina Santana (Hospital Clínic), Marta Tortajada (Hospital Clínic) and Antoni Trilla (Hospital Clínic–ISGlobal), the document assesses what can happen this winter in the northern hemisphere with the simultaneous circulation of COVID-19 and flu viruses, and what can we do to prevent the flu, prevent co-infection and ease the strain on the health system.
This year, the circumstances under which we will be facing the flu virus will be completely different from other years. We are in the midst of a second wave of COVID-19. The health care system is under great strain and health professionals are exhausted. A major or even moderate flu epidemic could overwhelm a system that is already stretched to the limit, while also increasing overall morbidity and mortality.
COVID-19 presents a clinical picture very similar to that of influenza: fever, cough, general malaise and muscle aches, plus some more unusual symptoms (e.g. sudden loss of smell or taste). Symptoms are generally mild, although 15% of patients may experience complications requiring hospital admission and 5% may have more serious complications requiring admission to an intensive care unit. The mortality rate of COVID-19 is about 1%, or ten times that of the flu.
Some theories suggest that the simultaneous circulation of two respiratory viruses could lead to the displacement of one of them. Therefore, it could be that the stronger virus will prevail—in other words, SARS-CoV-2 could displace the influenza virus. However, this speculation may not bear out in practice. For example, during the 2009 influenza pandemic, two flu viruses—H1N1 and H3N2—circulated simultaneously.
The outlook for this year is uncertain: faced with two viral infections that target the same organs (lungs and respiratory tract), have the same risk factors in terms of severity and complications, and share the same transmission route, what can we expect this coming winter?
We can theorise and try to extrapolate from what happened during the southern hemisphere winter, but we cannot guarantee that the same situation will repeat itself in the northern hemisphere.
Possible scenarios for the winter of 2020 in the northern hemisphere
- SARS-CoV-2 could displace the influenza virus, causing the latter not to circulate.
- Both viruses could circulate simultaneously, but with no interaction between them beyond a possible increase in the absolute number of cases of respiratory infection.
- The two viruses could interact and this interaction could lead to increased morbidity and mortality, in addition to an increase in the absolute number of cases.
- The influenza virus could undergo a major change (as yet not detected in the southern hemisphere) that causes the flu vaccine to be less effective, therefore leading to more influenza in addition to a large number of COVID-19 cases.
- Individual prevention measures against COVID-19 —physical distancing, hand washing, masks—could significantly limit the circulation of one or both of the viruses, even in the absence of more drastic measures such as lockdown.
However, this is only speculation. The only thing we know for sure is that we do not know what will happen. Therefore, we must not let down our guard: it is essential to maintain epidemiological surveillance of both viruses—as well as other viruses that are common in winter, such as RSV, in both children and adults—while also implementing prevention and control measures, which are generally the same for all viruses.
- The immediate future is uncertain. We do not know how SARS-CoV-2 will behave during flu season or how the influenza virus will behave in the age of COVID-19.
- The measures we have been applying to prevent COVID-19 are also effective at preventing influenza. Masks, physical distancing and hand hygiene must be our mantra in the coming months.
- Co-infection with both viruses is unlikely but possible and can increase the risk of complications and death.
- To prevent the flu, prevent co-infection and ease the strain on the health system, it is very important to get vaccinated. This is particularly important for people with risk factors—e.g. age over 60-65 years, underlying conditions and pregnancy, among others—as well as other essential groups that can transmit influenza, such as health care workers and teachers.
- The flu vaccine does not increase the risk of infection, complications or death from COVID-19.