When following up on the interactions of a Covid-infected patient, contact tracers in Switzerland track back to 48 hours before symptoms appeared. This misses up to 40% of transmissions, Zurich scientists have realised.

Sebastian Bonhoeffer from the federal technology institute ETH Zurich told Swiss public broadcaster SRF on Monday that the University of Hong Kong study behind the 48-hour figure was based on miscalculations.

Bonhoeffer was commenting on a report, published last week in the Swiss Medical Weekly, showing that tracing contacts for two days before symptoms appear catches just 61% of transmissions – not the 98% previously thought.

It would take up to five days of tracing back to ensure such a comprehensive overview.

“This was an extremely important study, which also led to the European Centre for Disease Control and the World Health Organization advising the 48-hour track-back,” Bonhoeffer told SRF. But this is “based on an error, we must trace contacts back further”.

He said the government’s Covid-19 Taskforce (of which he is a member) and the Federal Office of Public Health will discuss the issue and decide whether there is a need to update guidelines. He added that “this is not something that can or should be changed within just a few days”.

Contact tracing in Switzerland is carried out by the 26 cantons. So far, SRF reports, only Uri has reacted to the study and widened its tracing net from 48 to 96 hours.

Other cantons are waiting for a national decision, SRF says. Most of the regions also said that their contact tracing capacities were not overloaded and could “possibly” be expanded to trace contacts back further.

A notable exception was canton Geneva, where case numbers - and levels of contact tracing - are currently high.