Since the identification of Coronavirus disease MESHD 2019 (COVID-19) caused by severe acute respiratory syndrome MESHD coronavirus 2 (SARS-CoV-2) in China in December 2019, there have been more than 17 million cases of the disease MESHD in 216 countries worldwide. Comparisons of prevalence SERO estimates between different communities can inform policy decisions regarding safe travel TRANS between countries, help to assess when to implement (or remove) disease MESHD control measures and identify the risk of over-burdening healthcare providers. Estimating the true prevalence SERO can, however, be challenging because officially reported figures are likely to be significant underestimates of the true burden of COVID-19 within a community. Previous methods for estimating the prevalence SERO fail to incorporate differences between populations (such as younger populations having higher rates of asymptomatic TRANS cases) and so comparisons between, for example, countries, can be misleading. Here, we present an improved methodology for estimating COVID-19 prevalence SERO. We take the reported number of cases and deaths MESHD (together with population size) as raw prevalence SERO for the population. We then apply an age TRANS-adjustment to this which allows the age TRANS-distribution of that population to influence the case-fatality rate and the proportion of asymptomatic TRANS cases. Finally, we calculate the likely underreporting factor for the population and use this to adjust our prevalence SERO estimate further. We use our method to estimate the prevalence SERO for 166 countries (or the states of the United States of America, hereafter referred to as US state) where sufficient data were available. Our estimates show that as of the 30th July 2020, the top three countries with the highest estimated prevalence SERO are Brazil (1.26%, 95% CI: 0.96 - 1.37), Kyrgyzstan (1.10%, 95% CI: 0.82 - 1.19) and Suriname (0.58%, 95% CI: 0.44 - 0.63). Brazil is predicted to have the largest proportion of all the current global cases (30.41%, 95%CI: 27.52 - 30.84), followed by the USA (14.52%, 95%CI: 14.26 - 16.34) and India (11.23%, 95%CI: 11.11 - 11.24). Amongst the US states, the highest prevalence SERO is predicted to be in Louisiana (1.07%, 95% CI: 1.02 - 1.12), Florida (0.90%, 95% CI: 0.86 - 0.94) and Mississippi (0.77%, 95% CI: 0.74 - 0.81) whereas amongst European countries, the highest prevalence SERO is predicted to be in Montenegro (0.47%, 95% CI: 0.42 - 0.50), Kosovo (0.35%, 95% CI: 0.29 - 0.37) and Moldova (0.28%, 95% CI: 0.23 - 0.30). Our results suggest that Kyrgyzstan (0.04 tests per predicted case), Brazil (0.04 tests per predicted case) and Suriname (0.29 tests per predicted case) have the highest underreporting out of the countries in the top 25 prevalence SERO. In comparison, Israel (34.19 tests per predicted case), Bahrain (19.82 per predicted case) and Palestine (9.81 tests per predicted case) have the least underreporting. The results of this study may be used to understand the risk between different geographical areas and highlight regions where the prevalence SERO of COVID-19 is increasing most rapidly. The method described is quick and easy to implement. Prevalence SERO estimates should be updated on a regular basis to allow for rapid fluctuations in disease MESHD patterns.