The number of confirmed cases of Covid-19 in the US is increasing at an average of 41% per day, which could be driven by a lack of consistent infection control measures and an increase in testing.
Covid-19 cases caused by the novel coronavirus have been increasing exponentially in the US. On 16 March, the US reported just 4,400 cases of Covid-19. This has increased at an average of 41% per day for the last week to more than 41,000 confirmed cases as of the time of writing. Last week, the GlobalData epidemiology team conducted a three-scenario forecast model of Covid-19 transmission in the US, with a moderate estimate projecting more than 228,500 cases by 31 March. However, developments in the last week have shown the actual increase in confirmed cases was higher than the moderate estimate. After adjusting the model to take into account the higher daily increase rate of cases, there could be 733,000 confirmed cases by 31 March, almost three times as high as previously estimated. The adjusted model shows a grim two-week forecast of more than 8.1 million cases if the current rate holds.
Several factors could be driving the increase of Covid-19 cases in the US. The US is currently not exercising nationwide mitigation measures. Each state and county are responsible for determining when to close their events and gatherings. For many students in the US, last week was their spring break vacation, and the current onset of infections could be due to increased travel and gatherings. Additionally, some of the surges in cases could be due to improvements in testing. After weeks of delay, the US’ ability to test Covid-19 infections has started to improve. Some of the cases were previously “hidden,” and the improvements in testing means it is easier to see the true extent of infections in the country. However, the improvements in tests are slow to roll out uniformly around the country, and there are significant regional differences in access to tests. Many hospitals are only testing severe cases that require hospitalization, including those with a travel history or contact with known lab-confirmed cases. Limiting tests means that the confirmed case count significantly under-estimates the true case burden in the country. Currently, GlobalData epidemiologists believe that the number of reported confirmed cases in the US is mainly representative of the severe Covid-19 patients, which would only represent around 20% of actual people infected. This estimate matches well with the percentage of cases reported, as calculated using delay-adjusted mortality methods by Russell and colleagues published on GitHub on 22 March 2020. Given the count of 41,000 cases in the US at the time of writing on March 23rd, there are likely closer to 205,000 people infected in the US.
The multi-scenario model update for this week still shows a very large range in how the next two weeks could play out. Given the exponential increase of Covid-19 cases, early mitigation can have a huge impact on how many cases will occur two weeks later. The difference in the line representing the continuation of the past-week trend from the line representing moderate-risk scenario is only 2,600 cases on day one or 10% of the total confirmed case count. This is why early mitigation is key in controlling this outbreak. If cases can be decreased by 10% each day, then two weeks later it means a decrease of five million cases.