Keywords
SARS-CoV-2, COVID-19, seroprevalence, Malaysia
To the Editor,
Reported COVID-19 cases usually refer to acute infections diagnosed by
PCR and underestimate the true prevalence of disease. Seroprevalence
studies provide a more accurate picture as antibodies can be detected in
mild or asymptomatic cases who otherwise remain undiagnosed. The
majority of seroprevalence studies to date were carried out in developed
countries.
The first COVID-19 case in Malaysia was reported on January 25, 2020,
and the main wave occurred between early March and mid-April. With a
national movement control order instituted on March 18, aggressive
testing and public health measures, 8,354 cases had been reported as of
June 30, 2020,1 or 0.03% of the population. As of
June 6, most restrictions had been lifted as part of a phased recovery.
We aimed to determine SARS-CoV-2 seroprevalence in residual serum
samples collected at a teaching hospital serving Kuala Lumpur and
Selangor state, which together have reported 4,483 cases (51.9% of
national cases), or 0.05% of the combined
population.1
We retrieved 816 serum samples sent for diagnostic testing for
non-respiratory infections (mainly dengue) and archived at -20°C. These
were divided into periods according to dates of collection: pre-pandemic
(June-August 2019, n=228), main wave (January 29 to April 14, 2020,
n=327) and post-wave (April 15 to June 6, 2020, n=261). For each period,
between 17-65 samples were included from every 10-year age group
(<10, 10-19, 20-29, 30-39, 40-49, 50-59, 60-69, and
>70 years). The samples were from 368 females and 448
males.
Samples were first screened with an in-house indirect ELISA detecting
IgG to SARS-CoV-2 receptor binding domain (RBD), and shown to be 100%
sensitive for samples collected from 14 days post-onset of
illness.2 Screen-seropositive samples were confirmed
with a highly sensitive and specific (99.3-100%) surrogate viral
neutralization test (sVNT; cPass, GenScript, USA) based on total
antibody-mediated blockage of ACE2 receptor-RBD
interaction,3,4 which has received provisional
authorisation from the Singapore Health Sciences Authority. A two-step
testing process of screening and confirmation is useful for
low-prevalence settings where seropositives have a low predictive
value.4
These two assays were evaluated in our laboratory with the 228 (ELISA)
or 26 (sVNT) pre-pandemic serum samples as negative controls and 35
samples collected from PCR-confirmed COVID-19 patients at least 16 days
post-onset of illness. Sensitivity and specificity rates for the
screening ELISA were 97.1% and 88.6%, respectively. For the
confirmatory sVNT assay, sensitivity and specificity rates were 100%,
after increasing the inhibition cut-off from 20% to 25%, as suggested
by the manufacturer after assessing background reactivity in our
setting. Crude seroprevalence rates are reported with 95% exact
binomial confidence intervals (CI) approximated with Poisson
distribution. This study was approved by the University Malaya Medical
Centre medical ethics committee (no. 2017116-5794).
A total of 46 (7.8%) main wave and post-wave samples screened positive,
of which 3 were confirmed by sVNT. Two were from the main wave
(seroprevalence 0.6%; 95% CI, 0.07-2.2%) and 1 from the post-wave
period (0.4%, 95% CI, 0.01-2.1%) (Figure 1). Two were from males aged
in their 20s with previous diagnoses of COVID-19. The third was from a
65-year-old man with a 7-day history consistent with COVID-19, who was
not tested for SARS-CoV-2. As rates for the main wave and post-wave
periods were similar, they were combined to give a crude seroprevalence
rate of 0.5% (95% CI, 0.1-1.5%). Using 2019 age- and
gender-stratified population data
for Kuala Lumpur and Selangor from
the Department of Statistics, Malaysia
(http://pqi.stats.gov.my/searchBI.php), a direct age-standardised
seroprevalence rate was calculated as 0.4% (95% CI, 0-0.93%).
This study is potentially limited by bias arising from use of residual
inpatients serum. However, residual serum does provide similar estimates
of seroprevalence to cohort studies5 and is a
convenient option when preliminary data is needed during a lockdown. The
rate may also be underestimated because antibodies may take 2 weeks to
appear and may be undetectable in mild or asymptomatic cases.
The age-standardised seroprevalence of 0.4% for Kuala Lumpur and
Selangor found in this study is higher than the period prevalence of
confirmed cases of 0.05%. This is consistent with other seroprevalence
studies revealing >5 times more COVID-19 infections than
are reported.6 As this was a single centre study, a
more extensive national serosurvey is necessary to confirm our
preliminary indication that Malaysia has experienced limited SARS-CoV-2
transmission to date. With little herd immunity, Malaysia remains highly
susceptible to COVID-19 as we emerge from lockdown. Continued vigilance
in surveillance and public health measures are critical pending
availability of an effective vaccine.