COVID-19 testing can identify the SARS-CoV-2 virus and includes methods that detect the presence of virus itself (RT-PCR and isothermal nucleic acid amplification) and those that detect antibodies produced in response to infection. Detection of antibodies (serology) can be used both for diagnosis and population surveillance. Antibody tests show how many people have had the disease, including those whose symptoms were minor or who were asymptomatic. An accurate mortality rate of the disease and the level of herd immunity in the population can be determined from the results of this test. However, the duration and effectiveness of this immune response are still unclear.Due to limited testing, as of March 2020 no countries had reliable data on the prevalence of the virus in their population. As of 23 April, the countries that published their testing data have on average performed a number of tests equal to only 1.2% of their population, and no country has tested samples equal to more than 13.2% of its population. There are variations in how much testing has been done across countries. This variability is also likely to be affecting reported case fatality rates, which have probably been overestimated in many countries, due to sampling bias.

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