WASHINGTON - New analysis of SARS-CoV-2 genomes from a small number of California-based patients suggested the virus arrived in northern California through a complex series of introductions, including state-to-state transmission and international travel, according to a study published on Science magazine on Monday.

The research team led by Xianding Deng, researcher at the Department of Laboratory Medicine, University of California, San Francisco, used a method to assemble viral genomes directly from clinical samples, to better understand the nature of SARS-CoV-2 transmission in northern California through mid-March.

They analyzed positive viral samples recovered from 36 California-based patients spanning 9 counties and who had traveled on the Grand Princess cruise ship during its voyages associated with outbreaks.

Sequencing the viral genomes and placing them, along with other publicly available genomes, on a phylogenetic tree, revealed that the genomes from northern California were dispersed across the evolutionary tree of SARS-CoV-2, including lineages circulating in Europe and New York and related to early lineages from China, according to the study.

Based on their full results, researchers believed no one virus lineage was predominant in northern California, which suggests that transmission between communities was limited. Virus introductions to the state were likely from people traveling from outside the studied counties and the state, according to the study.

These results emphasized the need for social distancing and travel restrictions to contain SARS-CoV-2 spread in California and in other states, said the researchers.