1. Introduction
A pandemic respiratory disease is one of the scariest diseases due to its rapid spread among immunologically naïve humans and due to the fact that there is no vaccine against a new virus strain with a zoonotic origin. In the past two decades, there have been several outbreaks of zoonotic origin in human populations including the chikungunya outbreak in Caribbean countries and South America in 2013–2014 [1], Zika outbreak in the Americas in 2015–2016 [2], Ebola outbreak in 2014–2016 in West Africa [3], HPAI and LPAI outbreaks several times in many countries including Egypt, China, Indonesia, Canada and Australia [4,5], SARS-CoV outbreak in 2002–2004 in 26 countries [6,7], and MERS-CoV outbreak since 2012 in 27 countries [8]. However, a pandemic had not occurred until airborne viruses, including a quadruplex influenza A (H1N1) virus of swine origin and SARS-CoV-2 [9], emerged. The new H1N1 virus caused the 2009 influenza pandemic in June 2009 [10]. SARS-CoV-2, which causes COrona VIrus Disease 2019 (COVID-19), became the first recorded coronavirus pandemic on 11 March 2020 [11]. There are various influenza viruses (IVs) and various coronaviruses (CoVs), which are grouped into 4 genera, alpha, beta, gamma and delta IVs/CoVs (Table 1 and Table 2).
Of the three genera of IVs, alpha IVs (IAVs), beta IVs (IBVs) and gamma IVs (ICVs), that infect humans, eight segmented (−)ssRNA-containing IAVs (A/H3N2/68 and A/H1N1/09 variants) and IBVs (B/Yamagata and B/Victoria lineage-like viruses) with hemagglutinin (HA) spikes cause seasonal influenza epidemics that spread rapidly and cause mild to severe or fatal illnesses. Seasonal influenza vaccines and anti-influenza drugs against IAVs and IBVs are available, but seasonal influenza still causes up to 5 million severe illnesses and up to 650,000 deaths each year [12]. Thus, epidemics of IAVs and IBVs are important public health issues [12]. Seven segmented (−)ssRNA-containing ICVs with hemagglutinin-esterase-fusion (HEF) spikes usually cause mild upper respiratory disease but can cause lower respiratory disease in children and severe illness in infants [13]. Other seven segmented (−)ssRNA-containing IDVs with HEF spikes mainly infect cattle and cause respiratory illness [14]. These IDVs seem to have a zoonotic potential to infect humans, but whether they can cause illness in humans remains unknown [15]. Among all known influenza viruses, only IAVs are subtyped according to their HAs (H) and neuraminidase (NA or N) spikes into H1–H18 and N1–N11. Several subtypes of IAVs, including H1N1, H3N2 and H5N1, have crossed the species barrier to infect a variety of birds and mammals including humans, indicating that they have zoonotic potential. IAVs not only mutate quickly but also prefer to reassort with other IAVs to form a new strain. Due to these properties, IAVs have caused four pandemics in the past 102 years [10,16]. These viruses continued to threat human health seasonally. However, the first recorded pandemic virus, A/H1N1/1918-derived virus, disappeared from human circulation after the appearance of the A/H2N2/1957 pandemic virus containing five gene segments (PB2, PA, NP, M and NS) from human A/H1N1/1918-derived virus. Similarly, A/H2N2/1957-derived virus disappeared after the appearance of A/H3N2/1968 pandemic virus containing six gene segments (PB2, PA, NP, NA, M and NS) from human A/H2N2/1957-derived virus. In 1977, A/H1N1/1918-derived virus reemerged, probably due to accidental release from a laboratory, as a low-grade A/H1N1/1977 pandemic virus that primarily affected young immunologically naïve people [17,18] who were born after the end of the H1 period (1957 and later). This resulted in two subtypes of IAVs circulating in humans, A/H1N1/1977-derived and A/H3N2/1968-derived viruses. After the appearance of the A/H1N1/2009 pandemic virus containing one human gene segment (PB1) from A/H3N2/1968-derived virus and three gene segments (H1, NP and NS) from classical swine A/H1N1 virus believed to have been transmitted from A/H1N1/1918-derived virus between 1918 and 1920, A/H1N1/1977-derived virus disappeared from human circulation [4,19]. The disappearance of that virus resulted in only A/H1N1/2009-derived and A/H3N2/1968-derived viruses remaining in circulation in humans. Thus, IAVs are important not only because human IAVs cause seasonal influenza but also because nonhuman IAVs cause farm animal diseases, sporadic zoonotic outbreaks and periodic unpredictable pandemics.
Of the four genera of single linear (+)ssRNA-containing CoVs, only αCoV members (HCoV-229E and HCoV-NL63) and βCoV members (HCoV-OC43, HCoV-HKU1, SARS-CoV, MERS-CoV and SARS-CoV-2) have so far been reported to infect humans. HCoV-229E, HCoV-NL63, HCoV-OC43 and HCoV-HKU1 circulating in humans usually cause mild upper respiratory diseases [20], and there is still no vaccine or antiviral drug against these HCoVs [21]. Zoonotic SARS-CoV, a highly lethal CoV causing SARS disease, emerged in 2002. However, no SARS cases were reported after 29 April 2004 and the World Health Organization (WHO) officially announced on 18 May 2004 that the SARS outbreak had been contained [22]. Zoonotic MERS-CoV, another highly lethal CoV causing MERS disease, emerged in 2012. MERS cases are still being reported to WHO [8]. An anti-MERS drug and MERS vaccine are still not available. Zoonotic SARS-CoV-2 emerged in December 2019 and is causing the ongoing COVID-19 pandemic [11]. Many efforts are being made to develop COVID vaccines and anti-COVID drugs. As in the case of IAVs, CoVs have been isolated from a wide range of host species, and some of the isolated CoVs are zoonotic with the potential to cause an unpredictable pandemic. CoVs must therefore be controlled (Table 2).
The structures, diversities, host ranges and receptors of IVs and CoVs are summarized in this review. The host range of an infection depends on specific interactions between the virus strain and the host species. We will critically review current knowledge of receptor binding specificity, a crucial determinant of the host range, of IVs and human-infecting CoVs with appropriate viral historical perspective.