New York: According to a new report, some virus RNA may be reverse transcribed and incorporated into the human genome, which may explain why some people who had recovered from Covid-19 would test positive on a PCR test week or even months later.

Genetic sequences from the RNA virus SARS-CoV-2 will be incorporated into the genome of the host cell via a process called reverse transcription, according to researchers from the Whitehead Institute for Biomedical Research and Massachusetts Institute of Technology (MIT) in the United States. These segments of the genome can then be "read" into RNAs, which can then be detected by a PCR test. The findings were presented in the Proceedings of the National Academy of Sciences online journal.

The SARS-CoV-2 virus isn't the only one that has inserted itself into the human genome. The remains of ancient viruses account for about 8% of our DNA. In order to replicate, some viruses, known as retroviruses, depend on incorporation into human DNA.

"SARS-CoV-2 is not a retrovirus, which means it doesn't need reverse transcription for its replication," said first author Liguo Zhang, a post-doctoral researcher at Whitehead Institute. "However, non-retroviral RNA virus sequences have been detected in the genomes of many vertebrate species, including humans." Zhang added.

With this in mind, the researchers examined published datasets of RNA transcripts from a variety of samples, including Covid-19 patient samples, to see whether the novel coronavirus may be causing viral integration.

They were able to determine the percentage of genes transcribed in these patients' cells that contained viral sequences derived from integrated viral copies.

While the percentage ranged from sample to sample, it appears that a significant portion of viral transcripts were transcribed from viral genetic material incorporated into the genome in some cases. The frequency of SARS-CoV-2 integration in humans is still uncertain.

"The fraction of cells which integrate could be very small," said Rudolf Jaenisch, professor of biology at MIT. "But even if it's rare, there are more than 140 million people who have been infected already, right?” Jaenisch said.