The authorities screened 188,000 migrant workers staying at 43 workers’ hostels in Singapore in the second quarter of 2020.
The RT-PCR test was used to detect Covid virus RNA in nasal-pharyngeal swabs, and a blood test was done to screen for antibodies against Covid. The RT-PCR test was to diagnose cases of active Covid infection, while the antibody test was to identify those who had been infected in the past.
Out of the 188,000 migrant workers tested, 110,280 or 58.5% were positive for one or both of these tests. Of particular significance is that only 24,197 or 21.7% of those positive had ever been diagnosed as having Covid. The majority had been asymptomatic or had experienced symptoms so mild they did not go for medical attention. Almost four out of the five of them! (JAMA 2021; 325(6): 584-585)
We are now trying to reach herd immunity in Malaysia – as quickly as possible. The percentage of immunised people in the population that would confer herd immunity depends on the transmissibility of the virus. If the virus can spread to four susceptible people on the average (at a certain level of precautions), then we would need over 75% of the population to be immunised to forestall the possibility of exponential growth of infected people.
At 75% immunity, only one of the four persons who get an infective dose of the virus would develop an infection. The other three would be protected by the anti-Covid antibodies they have in their system.
Obviously, if R-naught (the rate of transmission or RoT) is higher, then we would need a higher level of immunity to attain herd immunity. For example, if a particular Covid strain had an RoT of 6, we would need 83% of the population to have immunity to forestall an exponential increase in infections.
The population of Malaysia is now 33 million, comprising citizens and permanent residents. We also have six million migrant workers in our midst. If we wish to attain herd immunity (and assuming the RoT is 5) we would need to get at least 80% (or 31.2 million) of the people living in Malaysia immunised. We must remember that there are between eight to nine million children and adolescents below 18 who are not being targeted for immunisation.
This is where the blood test for antibodies against Covid comes in useful – it would identify the individuals who have already developed immunity to Covid ‘naturally’ by undergoing an asymptomatic infection.
Such people might require vaccination to boost their immunity in the future – this we have to work out by observing whether they become susceptible to Covid after some time. But for now, we could move them to the bottom of our vaccination list.
It would be much more beneficial to us as a society to vaccinate people who are immunologically naïve – who have no anti-Covid antibodies at all. This is similar to the decision taken by some countries to give the first dose of the vaccine to more people instead of using the limited supply to administer the second dose to those who have already got the first.
The antibody test is available in Malaysia: it costs RM60 in private laboratories. It would be about a third of that if done in-house by the Ministry of Health.
But is it worth the effort to screen the entire population for the presence of antibodies against Covid? It would take effort as it is a separate initiative, and our health staff are already stretched with their current duties. Antibody screening would only help streamline our vaccination programme if a significant percentage of people – over 10% of the population in that locality – are positive.
To find out whether the antibody test would be of use anywhere in the country, we could do a quick survey. Every day, government hospitals draw blood from thousands of patients for various tests. We could introduce a protocol whereby a small percentage of these blood specimens are tested for antibodies against Covid as well. Over a few days, we would have got enough specimens to make a meaningful analysis.
The national master list of diagnosed Covid cases in Malaysia can be used to identify the antibody-positive cases who had never been diagnosed as a Covid case in the past. When this measure was carried out in New York State in mid-2020, they found that 25% of the people tested in certain districts of New York State were antibody positive.
If there are regions in the country where a significant percentage of the population have had undiagnosed Covid infections, then we could tweak our vaccination programme in those regions to include an antibody test 10 days before the vaccination date. Those testing positive for the antibody could be given a certificate stating that fact, and their place in the vaccination queue could be given to someone else who has no antibodies.
Antibody screening should also be done for our migrant workers. I would expect many of them in Selangor and Johor to be positive for Covid antibodies. That would help the authorities target those who are antibody-negative for early vaccination.
These steps would enable us to more rapidly reach our target of ensuring 80% of the people have antibodies against Covid.