A key limitation in our understanding of the COVID-19 pandemic is that we do not know the
true number of infections. Instead, we only know of infections that have been confirmed by a test – the confirmed cases. But because many infected people never get tested,
2 we know that confirmed cases are only a fraction of true infections. How small a fraction though?
To answer this question, several research groups have developed epidemiological models of COVID-19. These models use the data we have – confirmed cases and deaths, testing rates, and more – plus a range of assumptions and epidemiological knowledge to estimate true infections and other important metrics.
The chart here shows the mean estimates of the true number of daily new infections in the United States from four of the most prominent models.
3 For comparison, the number of confirmed cases is also shown.