The COVID antibody test might help us reach herd immunity faster

2021-06-04 12:10:08 0 Comments 39035 view

By Dr. Jeyakumar Devaraj

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-19 virus RNA in nasal-pharyngeal swabs and a blood test was done to screen for antibodies to Covid-19.

The RT-PCR test was to diagnose cases of active Covid-19 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-19. The majority had been asymptomatic or had had symptoms that were so mild they did not go for medical attention.

Almost 4 out of the 5 of them! (from Prevalence and Outcomes of SARS-CoV-2 Infection Among Migrant Workers in Singapore- in the Journal of the American Medical Association. )

The population of Malaysia is now 33 million, comprising citizens and permanent residents. In addition we also have 6 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 residing in Malaysia immunized.

We must remember that there are between 8 to 9 million children/adolescents below the age of 18 years who are not being targeted for immunization.

We are now trying to reach herd immunity in Malaysia – as quickly as possible. The percentage of immunized people in the population that would confer herd immunity depends on the transmissibility of the virus.

If the virus can spread to 4 susceptible people on the average (at a certain level of precautions), then we would need more than 75% of the population to be immunized to forestall the possibility of exponential growth of infected people.

For at 75% immunity, only one of the four person who get an infective dose of the virus would develop an infection. The other three would be protected by the anti-Covid antibodies that 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 strain of Covid-19 had a RoT of 6, we would need 83.4% of the population to have immunity to forestall an exponential increase in infections.

This is where the blood test for antibodies to Covid-19 comes in useful – it would identify the individuals who have already developed immunity to Covid-19 “naturally” by undergoing an asymptomatic infection.

Such people might require vaccination to boost up their immunity in the future – this we have to work out by observing whether they become susceptible to Covid-19 after a period of 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 obtained the first.

The antibody test is available in Malaysia – it costs RM 60 in the private laboratories. It would cost 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 Covid-19 antibodies? It will take effort as it is a separate initiative, and our health staff are already greatly stretched with their current duties.

Antibody screening would be only help streamline our vaccination programme if a significant percentage of people – more than 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 Covid antibodies as well. Over a few days we would have obtained enough specimens to make a meaningful analysis. 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.

The national master list of diagnosed Covid cases in Malaysia can be used to identify the antibody positive cases who have never been diagnosed as a case of Covid in the past.

If there are regions in the country where a significant percentage of the population have had undiagnosed Covid-19 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 opened up for another person (who has no antibodies).

Antibody screening should also be done for our migrant workers. I would expect many of them in Selangor and Johor will 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 our population have antibodies to Covid-19.