COVID-19 is not influenza
Authors: Eskild Petersen
Journal: The Lancet Respiratory Medicine (2020)

COVID-19 is often compared to influenza. In the middle of a pandemic with a new coronavirus transmitted from the respiratory tract, it is obvious to look at previous influenza pandemics and seasonal influenza for comparison. Yet it is important to understand that COVID-19 is not influenza. During the COVID-19 pandemic, several countries have struggled with overburdened intensive care unit capacity, whereas during the H1N1 pandemic in 2009, intensive care unit capacity was sufficient. For example, influenza never exceeded 4·5% of the total national intensive care unit (ICU) bed capacity in Denmark.1Gubbels S Perner A Valentiner-Branth P Molbak K National surveillance of pandemic influenza A(H1N1) infection-related admissions to intensive care units during the 2009–10 winter peak in Denmark: two complementary approaches.Euro Surveill. 2010; 1519743Crossref PubMed Scopus (15) Google Scholar In the spring of 2020, mortality for COVID-19 in Lombardia, Italy, reached 159 per 100 000 population.2Petersen E Koopmans M Go U et al.Comparing SARS-CoV-2 with SARS-CoV and influenza pandemics.Lancet Infect Dis. 2020; 20: e238-e244Summary Full Text Full Text PDF PubMed Scopus (858) Google Scholar By contrast, a study of influenza deaths during the 2009 pandemic estimated the all-age mortality in the USA to be 4·1 per 100 000.3Simonsen L Spreeuwenberg P Lustig R et al.Global mortality estimates for the 2009 influenza pandemic from the GLaMOR project: a modeling study.PLoS Med. 2013; 10e1001558Crossref PubMed Scopus (327) Google Scholar In The Lancet Respiratory Medicine, Piroth and colleagues4Piroth L Cottenet J Mariet A-S et al.Comparison of the characteristics, morbidity, and mortality of COVID-19 and seasonal influenza: a nationwide, population-based retrospective cohort study.Lancet Respir Med. 2020; (published online Dec 17.)https://doi.org/10.1016/S2213-2600(20)30527-0Summary Full Text Full Text PDF PubMed Scopus (372) Google Scholar report results of a retrospective study using data from the French national administrative database (PMSI), which includes discharge summaries for all public and private hospital admissions in France. The study compared 89 530 patients with COVID-19 admitted to hospital in March or April, 2020, with 45 819 patients with influenza admitted during the seasonal influenza outbreak between December, 2018, and February, 2019. The large sample size is an important strength of the study and it is assumed that the indication for hospital admission in the two periods was the same and thus does not bias the results. The results of the study by Piroth and colleagues clearly show that COVID-19 is more serious than seasonal influenza. In-hospital mortality was 16·9% (15 104 of 89 530) for patients with COVID-19 and 5·8% (2640 of 45 819) for patients with influenza and thus the relative risk of death for COVID-19 was 2·9 (95% CI 2·8–3·0). Hospitalised patients with COVID-19 were more likely to develop respiratory distress, pulmonary embolism, and septic shock, but were less likely to develop myocardial infarction or atrial fibrillation.4Piroth L Cottenet J Mariet A-S et al.Comparison of the characteristics, morbidity, and mortality of COVID-19 and seasonal influenza: a nationwide, population-based retrospective cohort study.Lancet Respir Med. 2020; (published online Dec 17.)https://doi.org/10.1016/S2213-2600(20)30527-0Summary Full Text Full Text PDF PubMed Scopus (372) Google Scholar Only haemorrhagic strokes (and not other types of stroke) were more frequent among patients with COVID-19. The median length of stay in the ICU for COVID-19 was twice as long as for influenza (15 days [SD 14] vs 8 days [9]; p<0·0001).4Piroth L Cottenet J Mariet A-S et al.Comparison of the characteristics, morbidity, and mortality of COVID-19 and seasonal influenza: a nationwide, population-based retrospective cohort study.Lancet Respir Med. 2020; (published online Dec 17.)https://doi.org/10.1016/S2213-2600(20)30527-0Summary Full Text Full Text PDF PubMed Scopus (372) Google Scholar Patients with COVID-19 were more often obese or overweight, diabetic, hypertensive, and dyslipidaemic, whereas patients with influenza more often had chronic heart failure, peripheral vascular disease, chronic respiratory disease, cirrhosis, and deficiency anaemia. By contrast, people living with HIV were not over-represented or under-represented in the COVID-19 group. These results are consistent with a study from New York including 393 patients with COVID-19 reported in April, 2020,5Goyal P Choi JJ Pinheiro LC et al.Clinical characteristics of COVID-19 in New York City.N Engl J Med. 2020; 382: 2372-2374Crossref PubMed Scopus (1602) Google Scholar and a study including 10 021 patients with COVID-19 from Germany.6Karagiannidis C Mostert C Hentschker C et al.Case characteristics, resource use, and outcomes of 10 021 patients with COVID-19 admitted to 920 German hospitals: an observational study.Lancet Respir Med. 2020; 8: 853-862Summary Full Text Full Text PDF PubMed Scopus (562) Google Scholar The studies by Piroth and colleagues4Piroth L Cottenet J Mariet A-S et al.Comparison of the characteristics, morbidity, and mortality of COVID-19 and seasonal influenza: a nationwide, population-based retrospective cohort study.Lancet Respir Med. 2020; (published online Dec 17.)https://doi.org/10.1016/S2213-2600(20)30527-0Summary Full Text Full Text PDF PubMed Scopus (372) Google Scholar and others5Goyal P Choi JJ Pinheiro LC et al.Clinical characteristics of COVID-19 in New York City.N Engl J Med. 2020; 382: 2372-2374Crossref PubMed Scopus (1602) Google Scholar, 6Karagiannidis C Mostert C Hentschker C et al.Case characteristics, resource use, and outcomes of 10 021 patients with COVID-19 admitted to 920 German hospitals: an observational study.Lancet Respir Med. 2020; 8: 853-862Summary Full Text Full Text PDF PubMed Scopus (562) Google Scholar clearly show that the risk groups are those with common conditions, specifically diabetes, obesity, and hypertension, and older people. Therefore, it is the responsibility of all of us to prevent spread of COVID-19. Physical distancing and the use of face masks is obligatory for all of us until we have a vaccine rolled out for major parts of the population. We should know that we can be infected and can spread the virus without having symptoms, and therefore the rules apply to all. A surprising finding of the study by Piroth and colleagues4Piroth L Cottenet J Mariet A-S et al.Comparison of the characteristics, morbidity, and mortality of COVID-19 and seasonal influenza: a nationwide, population-based retrospective cohort study.Lancet Respir Med. 2020; (published online Dec 17.)https://doi.org/10.1016/S2213-2600(20)30527-0Summary Full Text Full Text PDF PubMed Scopus (372) Google Scholar was that, among patients younger than 18 years, the rates of ICU admission were significantly higher for COVID-19 than influenza. The need for intensive care was highest in patients with COVID-19 who were younger than 5 years (14 [2·3%] of 613 for COVID-19 vs 65 [0·9%] of 6973 for influenza), but mortality in the COVID-19 group was not higher than for influenza. Mortality was ten-times higher in children aged 11–17 years with COVID-19 than in patients in the same age group with influenza (5 [1·1%] of 458 vs 1 [0·1%] of 804). These findings are supported by a study of 4784 children and adolescents with COVID-19 from Brazil7Sousa BLA Sampaio-Carneiro M de Carvalho WB Silva CA Ferraro AA Differences among severe cases of SARS-CoV-2, influenza, and other respiratory viral infections in pediatric patients: symptoms, outcomes and preexisting comorbidities.Clinics (Sao Paulo). 2020; 75e2273Crossref PubMed Scopus (18) Google Scholar and a study of children and adolescents from Spain.8García-Salido A de Carlos Vicente JC Belda Hofheinz S et al.Severe manifestations of SARS-CoV-2 in children and adolescents: from COVID-19 pneumonia to multisystem inflammatory syndrome: a multicentre study in pediatric intensive care units in Spain.Crit Care. 2020; 24: 666Crossref PubMed Scopus (110) Google Scholar Clearly, COVID-19 is not an innocent infection in children and adolescents. It is important to realise however, that the two cohorts are different in one important aspect. The influenza cohort would have had some degree of immunity to influenza. Although less than 50% of the population are likely to have received influenza vaccination in the study year, in previous years, most people would have been exposed to seasonal and pandemic influenza. Therefore, it must be assumed that there was some residual immunity against influenza in the influenza cohort.9Linderman SL Ellebedy AH Davis C et al.Influenza immunization in the context of preexisting immunity.Cold Spring Harb Perspect Med. 2020; (published online Sept 28.)https://doi.org/10.1101/cshperspect.a040964Crossref PubMed Scopus (13) Google Scholar By contrast, severe acute respiratory syndrome coronavirus 2 is a new virus for which no one had any previous immunity. Therefore, the difference between the two disease groups could represent differences between people with some immunity compared with people with no immunity. If so, this study highlights the importance of individuals receiving influenza vaccination each year, and especially during the COVID-19 pandemic. Concerns have been raised over the long-term effects of COVID-19.10Yelin D Wirtheim E Vetter P Kalil AC Bruchfeld J Runold M et al.Long-term consequences of COVID-19: research needs.Lancet Infect Dis. 2020; 20: 1115-1117Summary Full Text Full Text PDF PubMed Scopus (213) Google Scholar The PMSI database covers only hospital admissions, but perhaps a sample of individuals with COVID-19 could be extracted and followed up for 6 months to provide data on long-term morbidity and mortality. The study by Piroth and colleagues has important messages for the upcoming COVID-19 vaccination. Clearly people with obesity, diabetes, and hypertension must be considered high-risk groups, but the results also show that children and adolescents must be offered immunisation, given that young people can also become severely ill. Even if health-care workers and people older than 65 years are prioritised for the first rounds of immunisations, children and adolescents should also be offered the vaccine when it becomes available. I declare no competing interests. Comparison of the characteristics, morbidity, and mortality of COVID-19 and seasonal influenza: a nationwide, population-based retrospective cohort studyThe presentation of patients with COVID-19 and seasonal influenza requiring hospitalisation differs considerably. Severe acute respiratory syndrome coronavirus 2 is likely to have a higher potential for respiratory pathogenicity, leading to more respiratory complications and to higher mortality. In children, although the rate of hospitalisation for COVID-19 appears to be lower than for influenza, in-hospital mortality is higher; however, low patient numbers limit this finding. These findings highlight the importance of appropriate preventive measures for COVID-19, as well as the need for a specific vaccine and treatment. Full-Text PDF