3. Results and Discussion
336 samples (62.9%) were tested positive for influenza virus, which involved into 230 single influenza A virus (IAV)-positive samples, 4 single influenza B virus (IBV)-positive samples, and 102 IAV and IBV co-infection samples. Unfortunately, we found no positive signal of SARS-CoV-2 in 534 samples.
Until now, the source of SARS-CoV-2 was questionable. The early human cases could be epidemiologically linked to the exposure of the Huanan Seafood Market in Wuhan City, but there were several early cases that did not have the exposure of this market (Huang et al., 2020). This indicated that the seafood market was not the only source of SARS-CoV-2 infection. The genetic analysis showed that SARS-CoV-2 was likely to be a bat-origin coronavirus, exhibiting high genome identity with a β-CoV from Rhinolophus affinis bats (Zhou et al., 2020). This virus may still need an intermediate host to transmit from bats to humans. However, the intermediate host transmitted from bats to humans has not been fully understood. Several research groups have provided evidence based on sequence analysis that suggests pangolins are potential hosts of SARS-CoV-2 (Lam et al., 2020). And their pangolins samples were collected from foreign countries smuggling into Guangxi province and Guangdong province of China during 2017 to 2019.
Based on the similar symptoms between the influenza and COVID-19, we retrospectively detected over than 534 throat swab samples collected in influenza epidemic season in Guangzhou, however, no SARS-CoV-2 was discovered by the method recommended by authority. The results indicated that SARS-CoV-2 had not emerged in humans in Guangzhou in January to May of 2019, though asymptomatic infection could occur in humans (Rothe et al., 2020), and suggested that IAV was still major respiratory pathogen responsible for respiratory disease in the influenza season. Another possibility was that the viral load in these throat swab specimens was limited, because SARS-CoV-2 infection starts at the lungs, not in the upper respiratory tract (Huang et al., 2020). And there have been some cases of false negatives for RT-PCR of COVID-19 reported recently (Li et al., 2020). The first case of COVID-19 was reported in Wuhan, China, but its primary source may not come from this city. We think that more influenza samples from other places in 2019 or before need to be screened for SARS-CoV-2.