We congratulate the authors of the recently published article in your journal on their efforts to raise public health concern for COVID-19 and dengue detection in countries where both now co-exist.Three cases were presented. One was described as the first report of dengue-COVID-19 co-infection based on positive results in dengue NS1 and IgM tests, dengue serotyping and detection of SARS-CoV-2 by qRT-PCR. The other two cases were diagnosed as dengue only by the same tests, but COVID-19 negative as SARS-CoV-2 qRT-PCR was negative. Sera from all three cases cross-reacted in COVID-19 rapid tests. For the first case, this was obvious as antibodies to both viruses were present.The authors concluded that the dengue sera in the last two cases gave "false-positive" results in COVID-19 tests (since they were SARS-CoV-2 negative by qRT-PCR) and explained that this "cross-reactivity" was due to antigenic similarities between these two viruses. However, we think that an alternative and equally plausible explanation could be that both these patients were silently exposed to COVID-19 beforehand, remained asymptomatic during viraemia for about three weeks or so (Zou et al., 2020) and subsequently turned SARS-CoV-2 negative on qRT-PCR testing, due to virus clearance/low abundance (Wajnberg et al., 2020). Meanwhile, they developed COVID-19 antibodies, which are detectable for several months after infection (Ibarrondo et al., 2020). In this phase, if they contracted dengue, the reactivity in COVID-19 rapid tests was not necessarily due to "cross-reacting" dengue antibodies but could also have been due to pre-existing COVID-19 antibodies (Clarke et al., 2020). This is particularly possible for samples collected in the time frame when both diseases are co-existent in the population.Our same argument holds for the two Singapore case reports (Yan et al., 2020), where in a reverse scenario, sera from COVID-19 RNA-positive patients showed reactivity in dengue rapid antibody tests despite being dengue PCR-negative. Singapore is highly endemic for dengue (Tan et al., 2019). Therefore, it cannot be ruled out that both elderly patients had contracted dengue in the past and had pre-existing dengue antibodies in their serum.Nevertheless, we concur with the authors that there do appear to be antigenic similarities between SARS-CoV-2 Spike and dengue envelope, as evident from our observation that archived dengue serum samples from 2017, pre-dating the COVID-19 pandemic, cross-reacted in COVID-19 rapid antibody tests (Biswas and Sukla, 2020;Nath et al., 2020Nath et al., , 2021 and later independently confirmed by others (Lustig et al., 2020).
ContributionSB drafted the letter and critically evaluated the final version. All co-authors have contributed in collecting supporting data and references; writing of the letter and in discussions to arrive at the final version.