Introduction
Coronaviruses are Nidovirales and belong to family of coronaviridae, and subfamily orthrocoronavirinae (Shereen, Khan, Kazmi, Bashir, & Siddique, 2020). Coronavirus shows crown-like spikes on their outer surface; therefore, it was named as “Coronavirus”. The Coronaviridae has the four subgroups Alpha, Beta, Gamma, and Delta (α, β, γ and δ) coronaviruses(Zhong et al., 2003). The newly emerged SARS-CoV-2 belongs to β-coronaviruses, which is a enveloped, positive (+) sensed RNA virus (Su et al., 2016). SARS-CoV-2 causes a highly contagious disease called Coronavirus Disease 2019 (COVID-19) (Adhikari et al., 2020). The phylogenetic studies shows that SARS-CoV-2 has high resemblance with Severe Acute Respiratory Syndrome virus (SARS-CoV) and the relatively poor resemblance with Middle East Respiratory Syndrome virus (MERS-CoV) (Shereen et al., 2020) (Wang et al., 2020). This is the 3rd zoonotic coronavirus outbreak in the 1st two decades of 21st century. However, this time disease is rapidly transmitted from person to person and have raised a global human concern (Xie & Chen, 2020). Human to human transmission through close contact, air droplets and asymptomatic incubation carriers within the families, friends and the community made it extremely contagious (Guo et al., 2020). By following exponential growth up to the end of January, 2020, outbreak reached to several countries grabbing considerable attention in the entire globe (Cucinotta & Vanelli, 2020). As of March 11, 2020, WHO announced the epidemic to a global pandemic because of 13 folds increase in cases outside the China and number of affected countries also increased 3 folds. A further increase was is expected (Cucinotta & Vanelli, 2020).
Unfortunately, the Chinese New Year festival “Lunar” holidays, which is celebrated on the beginning of new year, accord with the outbreak of COVID-19, is a best celebratory event of the year in China during which huge number of peoples’ migration occurred towards their home towns (Cai et al., 2020). Approximately 3 billion people in China made close trips over the 40 days’ travel period of “Chunyun”. About 5 million individuals travelled from Wuhan, the epicentre of the COVID-19 outbreak, before the implementations of the travel ban on January 23, 2020 (Zu et al., 2020). Therefore, a rapid expansion and spread of COVID-19 cases transmission observed within the China and outside the China (Yang, Lu, et al., 2020). This review article highlights the ongoing challenges of COVID-19, pathogenesis, diagnosis, current and future treatment strategies, immune responses and future perspectives.