Over the past few days, a significant proportion of Singapore's recent confirmed cases of Covid-19 have been "imported cases".

There seems to have been some confusion, however, about the meaning of the term "imported cases", with some misunderstanding it to refer to foreigners entering Singapore with the virus.

And so, we thought we'd trace the usage and meaning of the term "imported cases" by both the World Health Organisation (WHO) and the Ministry of Health (MOH).

Before we dive in, though, here are the basics to know:

What are imported cases?

To be clear, the term "imported cases" refers to Covid-19-positive individuals who entered Singapore, regardless of nationality — in other words, they can be Singapore citizens too.

So far, imported cases are not linked to existing clusters in Singapore, but are also not considered by MOH to be unlinked cases (those with no known origin of transmission).

Cases defined as "imported cases" by the Ministry of Health (MOH) have included Singapore citizens, Singapore residents, and foreigners on short-term visits. In fact, a large proportion of Singapore's imported cases are Singaporeans returning from overseas.

Term originally referred to cases coming out of Wuhan

But it's also possible Singaporeans have reason to be confused — the types of cases classified as "imported cases" by both WHO and MOH appear to have shifted over the course of the Covid-19 outbreak.

"Imported cases" was at first used to refer specifically to cases that originated from Wuhan, or the larger Hubei province, the original epicentre of the virus outbreak, before the virus moved into other countries by people who had recent travel history to that area.

This definition appears to have since expanded to include cases for which the source of the infection is outside the country the infected person brings it into.

The reason this happened is now evident, given the significant shift in the epicentre of the virus from China and other parts of the Asian continent to Europe, and increasingly, the United States, in late-February through now.

Now let's dig deeper by looking into the usage and definition of "imported cases" by the WHO and our health ministry, examining how MOH has used the term to describe cases in Singapore, and laying out a breakdown of all of Singapore's imported cases and where they are from.

The WHO's usage of "imported cases"

The WHO used the words "imported case" in their first situation report on Covid-19 dated Jan. 21.

The phrase was used to describe two cases of Covid-19 (back then known officially as the very clunky "novel coronavirus", or "2019-nCoV") from Wuhan, Hubei in China, that had been reported in Thailand and Japan on Jan. 13 and Jan. 15 respectively.

From Jan. 21 to Feb. 28, the term "imported case" appears to have been used by the WHO to refer to cases found in other countries that originated from the Hubei province.

In the WHO's situation report on Feb. 6, it said that "public health efforts to control the spread of disease in countries with imported cases depend critically on the ability to detect the pathogen quickly (emphasis ours)".

At the time of that report, the number of confirmed cases of the virus had reached 28,060 within mainland China, while there were only 216 outside it.

The WHO's risk assessment at the time for China was "very high". Meanwhile, that of the region and globally was "high".

WHO introduces "transmission classification", includes other countries as sources of virus cases

By Feb. 28, the WHO had began to include countries outside of China as likely places of Covid-19 transmission.

It also started labelling each of the countries with reported Covid-19 cases with a "transmission classification".

This coincided with the day that the WHO's risk assessment on the regional and global levels both changed from "high" to "very high".

The five transmission classifications are:

• Community transmission, defined by the WHO as "the inability to relate confirmed cases through chains of transmission for a large number of cases, or by increasing positive tests through routine screening of sentinel samples".
• Local transmission, which indicates that "the source of infection is within the reporting location".
• Imported cases only, used to indicate that "all cases have been acquired outside the location of reporting".
• Under investigation, signifying that the type of transmission has not been determined.
• Interrupted transmission, used for places where "interruption of transmission has been demonstrated".

Countries that have multiple types of the above categories of transmission are classified in the highest category for which there is evidence, said the WHO.

Just FYI, in its Mar. 15 situation report, Singapore was classified under "local transmission".

But its definition of "imported cases" is now clear.

MOH uses "imported cases" to refer to first few cases

Singapore's first confirmed case of Covid-19, reported in an MOH announcement on Jan. 23, was described as an "imported case".

The case was of a 66-year-old male Chinese national from Wuhan who arrived in Singapore with his family on Jan. 20.

In the press release, MOH added, "Given the high volume of international travel to Singapore, MOH expects to see more suspect cases and imported cases. We urge the public to remain calm and vigilant, and to adopt good personal hygiene practices."

Jan. 24 saw two more Chinese nationals from Wuhan, also referred to as "imported cases".

Jan. 26 and 29 would see another four individuals, all referred to as "imported cases" of Covid-19, and all of whom are Chinese nationals from Wuhan.

"This is consistent with our assessment that more imported cases are expected from Hubei province. There is currently no evidence of community spread in Singapore", added MOH in its press release.

The trend continued, with all of Singapore's confirmed cases until Jan. 30 involving Chinese nationals.

Imported cases include Singaporeans

It was not until Jan. 31 that a Singaporean (Case 15) was confirmed to have the virus.

Case 15, a 47-year-old female Singapore citizen who travelled to Wuhan with her family, and was one of the Singaporeans evacuated from Wuhan on Jan. 30, was referred to as an imported case.

However, other Singaporeans, such as Cases 22, 23, and 45 — all of whom acquired the virus in Wuhan and were subsequently evacuated back to Singapore — were not specifically categorised by MOH as "imported cases".

Imported cases from outside of China

On Mar. 5, the term "imported case" was used by MOH again in their announcement of new confirmed cases for the first time in a month.

However, this time, it was not used to describe a case acquired in China and subsequently brought into Singapore.

The imported case — Case 113 — is a 42-year-old male French national holding a Singapore work pass.

He had no recent travel history to affected countries and regions, but had been in France, Portugal and the United Kingdom between Feb. 8 and Mar. 3.

From that point, Singapore started seeing more cases that according to MOH were "likely to be imported", such as:

• Case 123, a 47-year-old male Singapore citizen who had been in South Africa from Feb. 14 to Feb. 17 and France from Feb. 22 to Feb. 24.
• Case 124, a 37-year-old female Singapore Permanent Resident (PR) who had been in Germany from Feb. 27 to Mar. 5.
• Case 132, a 37-year-old female Singapore PR who had been in London from Feb. 23 to Feb. 27
• Case 136, a 36-year-old male Italian national Singapore Work Pass holder who had travelled to the US from Feb. 7 to Feb. 22, and to Italy from Feb. 23 to Feb. 29.

Singapore also saw imported Indonesian cases who even experienced symptoms back home before they flew here to check into Singapore hospitals.

Case 147, for example, was a 64-year-old male Indonesian national who arrived in Singapore on Mar. 7. He was the first imported case involving a foreigner not on a Singapore work pass since Feb. 5.

Case 152, another Indonesian, arrived in Singapore on Mar. 7, but who had reported symptoms on Feb. 28 while in Indonesia and had sought treatment in Jakarta prior to arriving in Singapore.

Between Mar. 11 and Mar. 16, the majority of each day's new reported confirmed cases of Covid-19 were imported:

• 8 out of 12 (Mar. 11),
• 5 out of 9 (Mar. 12),
• 9 out of 13 (Mar. 13),
• 9 out of 12 (Mar. 14),
• 9 out of 14 (Mar. 15), and
• 11 out of 17 (Mar. 16).

Over the six days, 51 out of 77 total confirmed cases were imported cases.

In fact, the vast majority of these confirmed cases over the past six days involved Singaporeans returning from overseas — here's the breakdown:

• 24 are Singaporeans,
• three are PRs, and
• 16 are foreigners residing in Singapore.
• Only eight are foreigners on short-term visits.

1/3 of S'pore's imported cases are foreigners on short-term visits

In total, 81 cases so far have been described by MOH as imported cases.

Out of the 81, 30 of the imported cases are Singaporeans, five are Singapore PRs, 19 are foreigners residing in Singapore holding work passes or long-term visit passes (LTVPs). 27 are foreigners on short-term visits.

The bulk of them are actually people who live and work in Singapore, and unfortunately, brought back the virus when they went overseas.

Here's a breakdown of the imported cases up to March 16:

Singaporeans

Singapore PRs

Foreigners residing in Singapore

Foreigners on short-term visits

Measures taken to curb more imported cases

The fact that there has been a spike in imported cases in Singapore is not necessarily surprising, given the exponentially-rising numbers of cases in locations that Singaporeans and Singapore residents may choose to frequent, such as the U.S., the UK, France, other parts of Europe, and nearby Southeast Asian countries like Malaysia and Indonesia.

Thus, MOH has taken steps to try to slow down the importation of cases by announcing on Mar. 13 that they are barring short-term visitors from Italy, France, Germany, and Spain from entering Singapore.

The government has also issued a travel advisory for Singaporeans to defer all non-essential travel for the next 30 days.

All Singaporeans, Singapore PRs, and long-term pass holders returning to Singapore with recent travel history to Italy, France, Germany and Spain as well as previously-named countries will also now be issued with a Stay-Home Notice (SHN).

On Mar. 15, the government extended the SHN requirement to all travellers with recent travel history to Japan, Switzerland, the UK, and ASEAN countries.

So after looking at the details of the imported cases in Singapore, it is clear that, while imported cases are indeed something to be aware of and careful about, it might not be accurate to draw conclusions about foreigners taking advantage of Singapore's system based on the number of imported cases.

Short of barring Singapore residents from travelling overseas (as Malaysia is now doing), those who live and work here can help to prevent transmission of the virus by refraining from travelling to affected places.

If you are planning to travel during this time, perhaps it would be wise to ponder what one of our writers wrote in a commentary — to deeply consider whether and how you can do so responsibly.

Read more about the measures Singapore has taken and escalated to prevent further importation of cases during various parts of the virus outbreak:

Mothership Explains is a series where we dig deep into the important, interesting, and confusing going-ons in our world and try to, well, explain them.

This series aims to provide in-depth, easy-to-understand explanations to keep our readers up to date on not just what is going on in the world, but also the "why's".

Top image adapted from Changi Airport Facebook page, Straits Times Facebook page, and Mothership Facebook page.