How much COVID testing do we need: (3) Who do we need to test?

We have estimated that Sri Lanka needs to be doing 2,000–6,000 RT-PCR tests a day. We have since learnt that the MOH Epidemiology Unit estimates we need to be doing 5,000 tests a day. This is essentially the same number because our estimates include testing of airport arrivals, and the airport remains closed. We provide here additional explanation of why we need to test specific groups. We will include this in a revised version of the report. In providing this, we emphasize two points. 

The first is that Sri Lanka remains a country with minimal cases of Covid-19 virus. Since we have closed our borders, our immediate goal must be to clear the country of the virus, and then put in place a surveillance system to detect cases quickly and prevent future outbreaks, when we start to allow limited international arrivals. Rapid detection is also critical in order to slow transmission of the virus in the community by reducing the time that infected people spend infecting others. International arrivals are not something we can avoid for ever, since our citizens living overseas have a constitutional right to return. In the longer term, we will also want to allow some essential business and possibly tourist travel to help our economy.

The second point to stress is that there is broad consensus by experts and health authorities in the places (Singapore, Hong Kong, Taiwan), who have been most effective in controlling the virus, that this range of targets is the most appropriate and necessary given what we currently know.

All international arrivals

Most countries now require that all arrivals must go through 14 days quarantine—Sri Lanka will need to do this in future. Our current outbreak stems from the failure to quarantine all arrivals from early March. Unfortunately, 14 days quarantine will not catch all symptomatic infections. Chinese researchers had published data in early February indicating that the incubation period could be longer than 3 weeks [1, 2], and in early March other researchers using data from outside China concluded that 1% of all infected cases who develop symptoms will only do so after two weeks [3]. 

In addition, since Covid infection is asymptomatic in many cases, many infected people in quarantine will never develop symptoms anyway. Some of the recent cases in Sri Lanka fall into this category [4], and it should not have come as surprise to anyone. Until we have better options, we need to test all arrivals, including everyone who is currently in quarantine, before releasing them. 

Immediate contacts of detected cases

People in close contact with Covid cases have the highest risk of infection. During the curfew, most infection transmission will be within families. MOH policy is to ask these people to self-isolate at home for 14 days, and then test them if they develop symptoms. There are three risks with this policy: (i) Many infected people never develop symptoms but they can still infect others – these people will not be detected without testing; (ii) if there is more than one person living in a home, it is possible for the virus to circulate between them for much longer than 2 weeks; and (iii) we cannot assume that everyone will comply with isolation instructions. To be fair, many countries are not testing all immediate contacts, but most countries do not have the goal of zero community transmission, e.g. the UK and USA—Sri Lanka can achieve zero transmission and testing all immediate contacts is still feasible for us.

All ICU Admissions

Around 1% of people infected with Covid-19 become seriously ill and will end up in an ICU, even if they have not been diagnosed as having Covid-19. This makes ICUs the most likely place to find undetected infections and makes testing ICU patients one of the most cost-efficient ways of finding unknown cases. Singapore, Hong Kong and Taiwan all do this for this reason, and the Imperial College COVID expert group recommends that countries do this in order to provide early warning of a hidden outbreak [5]. 

Another reason to test all ICU admissions is that undetected Covid-19 cases are extremely dangerous. Covid-19 is so infectious, that they are likely to infect other patients and ICU staff. This is more likely in Sri Lanka, because few of our ICU beds are in single rooms which can be isolated. ICU patients are already ill, so Covid-19 infection is likely to kill many of them. Infection of ICU staff is also a big problem because they will have to go into isolation for two weeks, further reducing the ability of hospitals to provide ICU care.

All pneumonia and influenza admissions and deaths of unknown infectious origin

MOH now recommends—we don’t know if this is required—that all pneumonia admissions which have no obvious cause be tested. This is good, but Singapore, Hong Kong and Taiwan go further and test all such cases. The logic is simple. If the testing requirement is optional or subject to any discretion, there is a risk that some cases will go undetected. This is a real risk as there have been numerous instances around the world of Covid cases being admitted and misdiagnosed as normal pneumonia, and in several countries including Japan and USA, these cases were key factors in the initial epidemic. To prevent this happening in Sri Lanka, it should be obligatory to test all pneumonia admissions, and this should be also required in the private sector. This same logic also applies to testing deaths of unknown infectious origin, which is the policy again in Singapore and Hong Kong.

Outpatients with fever or respiratory symptoms

MOH currently recommends that outpatients with fever AND respiratory symptoms AND a history of international travel be tested. This is not sufficient to catch all cases. We have already had Covid patients with no history of foreign travel who went to multiple doctors before being admitted and being found to have the virus. We have also had several cases where recent arrivals have not disclosed their travel history. To avoid this risk and to minimize the delay in detecting cases, both Singapore and Hong Kong PCR test all outpatients who present with possible symptoms, and this has been successful in detecting unknown community transmission [6]. To make this more practical, Hong Kong asks these patients to go home and bring back a saliva specimen [7], which can be good enough [8], and it has also extended this scheme to private doctors recognizing that many outpatient consultations are not in the public sector [9].

Patients who don’t fit any criteria but where doctors suspect possible infection

No set of criteria can be foolproof. Both Singapore and Hong Kong have found that giving doctors discretion to refer patients for testing can catch additional cases, and if our goal is zero transmission, we need to allow for this too.