It is widely accepted that many of those infected with COVID-19 are asymptomatic and hence may not even figure in the official list of positive cases. A large ‘antibodies’ study commissioned by the Spanish government now reveals that the number of officially reported cases in Spain are only about 10% of the potentially infected population. As per latest available data, more than 70 lakh (7 million) cases of COVID-19 were recorded across the world. Conservative estimates peg the number of tests carried to identify these cases at more than 10 crores (100 million). Countries across the world have adopted different testing strategies to identify COVID-19 cases. The ambiguity surrounding the symptoms, means and extent of the spread of infection etc. have so far prevented from developing a standard/uniform strategy to detect the spread. Over the last few months, the understanding of COVID-19 has also changed & improved, largely on account of new research. Various governments have commissioned research studies to understand the spread much more in detail. In this story, we look at one such research study conducted in Spain regarding the prevalence of COVID-19 infection by detecting antibodies against the virus. The Sample size makes it one of the largest research studies regarding COVID-19 Spain is among the countries severely affected by COVID-19, both in terms of number of positive cases as well as fatalities with more than 27 thousand deaths. It is also one of the countries where initial outbreak of COVID-19 was widespread outside of China. It looks like the country has currently overcome the first wave of COVID-19 infection as the number of daily new cases has decreased substantially compared to a month ago. This research study named ENE-COVID 19 (National Sero-Epidemiology Study of SARS-COV-2 Infection in Spain) was carried out through a collaborative effort between Spain’s Ministry of Health and the Carlos III Health Institute, along with support from National Statistics Institute. A total of 60,983 participants took part in the first round of tests conducted in the two-week period of 27 April to 13May,2020. Of these, 86 participants were excluded from the analysis since they did not have details for all the demographic values considered by the study. Apart from these, there was another complementary island study carried out in islands not included in the national study. The number of valid tests from this complementary study is 3,234. The participant rate among the eligible individuals was 62.3%, while it was 74.7% among those who were contacted to take part in the test. ENE-COVID 19 is a Sero-epidemiological longitudinal study A Sero-epidemiological study is an epidemiological study which is based on the blood serum antibodies. ENE-COVID 19 study being carried out in Spain is based in Sero-epidemiological tests. The various COVID-19 tests carried out across the world are to detect the presence of the COVID-19 virus in the body, and to determine the need for isolation and treatment. On the contrary, an antibody test checks if there are any antibodies in the blood, which are typically formed when the person successfully fights off an infection like COVID-19. Testing positive in this test would mean that the person most likely had an infection earlier (not necessarily realize it) and has some degree of immunity to the infection. Identifying individuals who have developed antibodies can help in further research on developing cure for COVID-19. The results of such tests provide important insights into the spread of COVID-19 as many asymptomatic individuals may not have been tested at all. Two types of tests used for the study Based on the inputs from National Institute of Statistics, 1500 census units and 24 addresses were selected for the study in Spain. These units were selected taking into account different criteria like geographical units, density of population etc. From these units, a total of 90 thousand people was invited for testing and to be part of the study, of which around 60 thousand participated. The participants were further divided into two sub cohorts. In view of the geographical diversity and peculiarities of few of the islands, additional participants were considered as part of the sampling beyond the 60 thousand + participants. Two types of tests were used to measure anti-SARS-COV-2 (i.e. COVID-19) antibodies. The Rapid tests allow for getting the results on the spot and the blood sample is collected using a prick. The reliability studies carried out for ENE-COVID 19 study indicate a sensitivity of 73% and 79% respectively for IgM and IgG (Antibodies) and a specificity of 98% for the former and 100% for later.