High number of unlinked cases may not be worrying, say experts

The higher numbers of unlinked Covid-19 cases recently may not be a cause for concern as they reflect a slower identification of links between cases rather than widespread transmission, said experts.

There were 29 unlinked cases out of the 133 new Covid-19 cases yesterday. This comes after 52 unlinked cases were initially reported out of 170 new cases on Thursday - the highest number of unlinked cases in a single day so far.

The 52 unlinked cases had fallen to 29 by yesterday evening, according to the Ministry of Health's nightly virus update.

Overall, the seven-day moving average of unlinked community cases increased from 10.1 on Thursday to 13.9 yesterday.

Professor Teo Yik Ying, dean of the National University of Singapore's Saw Swee Hock School of Public Health, said that as the number of cases increases, the ability to complete contact tracing before each case is reported to the public is reduced.

"(Thus) we will naturally see more unlinked cases reported initially, which subsequently will be successfully identified (as stemming) from presently known clusters," he said.

Meanwhile, the slower identification of links in itself is not a cause for concern as the cases have already been detected and isolated, said Prof Teo, adding that linking the cases is therefore more for monitoring the situation in the community.

Agreeing, Professor Hsu Li Yang, vice-dean of global health at the Saw Swee Hock School of Public Health, said that when there is greater spread of Covid-19 in the community, the actual number of true unlinked cases will also increase.

Prof Teo added, however, that if there is a high number of cases linked and detected through surveillance testing but not yet isolated or quarantined, it would mean that the infected individuals could potentially have seeded new chains of transmission.

There were 78 such cases on Thursday and 37 yesterday - a downward trend from the 106 detected on Monday.

Prof Teo also pointed out that an unintended consequence of having more of the population vaccinated is the emergence of more unlinked cases. This is because a vaccinated person who is infected is likely to be silently transmitting the virus as he may have mild symptoms or no symptoms at all.

Prof Teo therefore expects unlinked Covid-19 cases to rise proportionately as Singapore continues to vaccinate its population.

More than 70 per cent of the population have received one dose of a Covid-19 vaccine, while about 50 per cent have been fully vaccinated.

If more of the reported cases are unvaccinated, or the number of infected people requiring hospital care starts to increase, then it would be more of a concern to public health, said Prof Teo.

In the last 14 days, there were 266 cases across all age groups who were unvaccinated. Among them were 32 children aged 11 and below, and 30 seniors aged 60 and above.

Among the 133 reported cases yesterday, two were seniors aged 70 and above, and they were unvaccinated or partially vaccinated, which means they are at risk of severe illness.

At this point, however, it is less important to establish links within the community than to slow down the spread of the virus to manageable levels until a large proportion of the population has been vaccinated, said Prof Hsu.

He said this explains why the multi-ministry task force tackling Covid-19 has decided to revert to phase two (heightened alert) to slow the transmission of the virus, in view of the fairly large number of unvaccinated elderly people.

Prof Teo said he expects that when Singapore is ready to embark on its endemic Covid-19 plan, public health interventions will be relaxed in phases. Contact tracing will be one intervention that may be modified or even ceased completely at a later stage.

"Like in cases of influenza, we do not really aim to identify where people are being exposed and infected, but rather whether these people subsequently go on to require hospital or more in-depth healthcare," he said.

"So, I expect we may eventually stop the entire process of contact tracing, but keep up with some degree of genomic surveillance in order to identify the dominant strains of the Sars-CoV-2 virus that are circulating in Singapore."