They should also get tested if suspect symptoms appear.
The case came to light when a 33-year old resident of Hong Kong passed through mandatory screening earlier this month at the Hong Kong airport on his way back from Europe. The so-called PCR swab test was positive.
This came as a surprise because the man had contracted — and recovered from — a COVID infection four-and-a-half months earlier, and was assumed to have immunity, especially after such a brief time since the infection.
To find out whether he had suffered a relapse or had been infected anew, To and his team sequenced the two virus strains and compared their genomes, or genetic coding.
The two viral signatures were “completely different”, and belonged to different coronavirus lineages, or clades.
The first closely resembled strains collected in March and April, and the second strain matched the virus found in Europe — where the patient had just been visiting — in July and August.
“The virus mutates all the time,” said To. “It is very unlikely that the patient would have gotten the second virus during the first infection.”
The fact that a blood sample — taken shortly after the positive test at the airport — showed no antibodies is a further indication that the second virus had not been lingering unnoticed for months.
“This is certainly stronger evidence of reinfection than some of the previous reports because it uses the genome sequence of the virus to separate the two infections,” said Jeffrey Barret, a senior scientific consultant for the COVID-19 Genome Project at the Welcome Sanger Institute, commenting on the study.