The clinical presentation and laboratory indices at hospital admission were common in H7N9 and H1N1 patients, except that productive hemoptysis, thrombocytopenia were more common in H7N9 patients, those two factors have been associated with more severe outcomes . Compared with features in patients with H7N9 and H1N1, patients with COVID-19 were more likely to exhibit the mild symptoms. Chinese Center for Disease Control and Prevention (China CDC) recently reported that most of the confirmed cases were classified as mild or moderate, 13.8% as severe, and only 4.7% as critically ill  Notably, the viral load that was detected in the asymptomatic patients was similar to that in the symptomatic patients  Recent research indicated that asymptomatic carriers can result in person-to-person transmission and should be considered a source of COVID-19 infection . Unlike H7N9 and H1N1, the transmission of COVID-19 occurs during the prodromal period when those infected were mildly ill, and carry on usual activities, which contributes to the spread of infection.
The mean duration of viral shedding for nasopharyngeal specimens was 9.8 days for SARS-CoV-2 vs 9.9 days for H7N9, for severe cases, it was 10.6 days vs 10.2 days. It has been widely investigated that the duration of viral shedding in H1N1 ranged from 4 to 8 days [13,14,15]. It seems likely that SARS-CoV-2 infections were characterized by the prolonged viral shedding, compared with H1N1. SARS-CoV-2 viral RNA has been detected in the serum, urine, and feces of COVID-19 patients, but it is not known if this represents viral replication occurring outside of the respiratory tract . Although the H7N9 patients had high mortality, there had been no confirmed cases of human-to-human transmission, and most infected humans had a history of contact with poultry or of having visited a wet market, and the outbreak of H7N9 had been controlled by integrative measures including the closedown of the wet markets in the affected areas [17,18,19]. However, the clinical spectrum of SARS-CoV-2 infection appears to be extensive, encompassing asymptomatic infection, mild upper respiratory symptoms, and spreading by respiratory and fecal–oral transmission.
According to the report of world health organization, the epidemic threshold of seasonal influenza was in December around the world, influenza activity decreased overall or returned to baseline levels in March for most temperate regions . Worldwide, seasonal influenza A (H1N1) viruses accounted for the majority of detections. According to the report of China CDC, the mortality of influenza A (H1N1) diseases was less than 0.01% during the epidemic threshold period in china . However, SARS-CoV-2 infections had a higher complication rate and more severe cases than H1N1 patients (Table 1). Furthermore, recent research reported that R0 of COVID-19 might be as high as 6.47 (95% CI 5.71–7.23) . Likely, SARS-CoV-2 activity would not decrease with the change of seasons; long-term control measures are still needed.
Neutrophilia was more common in H7N9 and H1N1 patients, compared with COVID-19 cases. It maybe implied that patients with H7N9 and H1N1 were more likely to develop secondary bacterial infections. H7N9 cases had a higher proportion of glucocorticoids and antibiotics therapy. Increasing the risk of secondary infection, glucocorticoids could delay the clearance of coronavirus nucleic acids without lower hospitalization-fatality ratio for patients with H7N9 diseases . Nevertheless, recent research suggested that timely and appropriate use of corticosteroids, together with ventilator support, should be considered for severe patients to prevent ARDS development . According to the chest CT images of COVID-19 patients on admission, the mainly positive findings were various ground-glass densification shadows with multiple diffusions in both lungs, the pros and cons need to be weighed carefully before antibiotics and glucocorticoids treatment.
The mean time from illness onset to severity was 8.0 days for COVID-19 and 5.2 days for H7N9, it may imply that there was a therapeutic window that could be exploited, provided comprehensive treatment including an active antiviral agent was available. But, if not treated promptly, the asymptomatic or mild cases may develop severe pneumonia, even end up dead. The therapeutical emphasis of COVID-19 was to antiviral early that would decrease the peak viral load and delay the progression of lung lesions; thus this would reduce the hospitalization-fatality ratio .The mean time from illness onset to severity for H7N9 was shorter than COVID-19, the key to controlling disease progression was early detection and timely treatment.
Chronic heart disease more frequent in patients hospitalized with COVID-19, compared with H1N1 (Table 1), whereas for severe cases, COVID-19 had the highest prevalence of chronic heart disease than H7N9, which might be associated with increased secretion of ACE2 in the COVID-19 compared with H7N9 and H1N1. SARS-CoV-2 infection is triggered by binding of the spike protein of the virus to ACE2, which is highly expressed in the heart and lungs . There have been hypothesized that the use of ACE-inhibitors and angiotensin receptor 1 blockers (ARBs) may have effect the course of COVID-19 [27, 28]. Nevertheless, further evidence are required before any recommendations are made about starting or withdrawing ACE-inhibitor and ARB medications.
The comparisons in this study are limited by a lack of parameters of the transmission dynamics. Further research is still needed if the epidemic features of COVID-19 are similar to Seasonal influenza A H1N1 or not.