The present invention provides the complete genomic sequence of a novel human coronavirus, coined as human coronavirus-HKU1 ("CoV-HKU1"), isolated in Hong Kong. The virus belongs to the order Nidovirales of the family Coronaviridae, being a single-stranded RNA virus of positive polarity. Further study on nasopharyngeal aspirates from patients with community-acquired pneumonia has revealed that there are two genotypes, genotype A and genotype B, for this virus. In addition to the genomic sequences of these two genotypes, the invention provides the deduced amino acid sequences of the complete genome of the CoV-HKU1. The nucleotide sequences and deduced amino acid sequences of the CoV-HKU1 are useful in preventing, diagnosing and/or treating the infection by CoV-HKU1. Furthermore, the invention provides immunogenic and vaccine preparations using recombinant and chimeric forms as well as subunits of the CoV-HKU1 based on the nucleotide sequences and deduced amino acid sequences of the CoV-HKU1.