Health officials have urged people not to report past infections to try to get an exemption from the government's vaccine pass scheme, as Hong Kong saw a surprise jump in new Covid cases on Wednesday, to 505.

The Centre for Health Protection (CHP) said every other indicator of the seriousness of the pandemic remains stable – including hospitalisation rates and the amount of virus found in sewage samples – and the highest daily Covid caseload in more than a month could involve many "false positive" cases.

The controller of the CHP, Dr Edwin Tsui, said there has been a surge in the number of people testing positive through rapid tests, but subsequently testing negative in a PCR test.

He said in the past couple of months, the daily false positive rate was around 7 percent, but in recent days, it has risen to as high as 38 percent.

He suggested that some people might be reporting past infections to obtain a QR code exempting them from having received three Covid jabs in order to be allowed into various premises across the city under the vaccine pass scheme.

"I would like to remind the public, if you didn't test positive on the day, don't use our self-report platform to report your case," Tsui said at the government's daily Covid press conference.

"If you want a recovery record or to satisfy the vaccine pass, the best way is to get jabbed, then you can fulfil the triple jab requirement."

Tsui added that officials are now following up all reported positive results from rapid antigen tests (RATs) with PCR tests and people found to have the Omicron sub-variants BA.2.12.1, BA.4 or BA.5 are being sent to a hospital or quarantine centre, along with their close contacts.

"The latest information shows that these Omicron sublineages may have higher transmissibility... we have to take a more stringent approach," he explained.

Among the latest infections, 34 involved people returning from overseas.

One more Covid-related death was reported, involving an 88-year-old woman who was chronically ill.