Cases of Covid-19 are surging across the United States sun belt, the region from southern California to Florida. More than 137,000 Americans have died, and more than 3 million have been infected.

But as cases have surged, daily deaths associated with the pandemic have not risen to the seismic numbers seen in April, when New York City was gripped by the virus.

More than 2,429 were reported dead due to coronavirus on 21 April nationally, one of a set of horrific days that month. On 17 July 977 people were reported dead, even as infections increase rapidly in the south and west.

Politicians have seized on this, including Donald Trump, who has perpetuated falsehoods about the death rate even as hospitals across the region begin to fill.

First, lets talk about what it does not the likelihood that an infected person will die. That measurement is called the case fatality rate, and is the reason we know Covid-19 causes far more severe disease and death than a typical flu.

Its very important to understand the lethality of a pandemic, said Dr Howard Markel, a pandemic historian, expert in infectious diseases and a pediatrician at the University of Michigan School of Public Health.

Covid-19 has a high case fatality rate, which is the reason authorities believe it is worth disrupting our lives to stop it. Like many metrics with Covid-19, the case fatality rate is based on preliminary data which may be incomplete.

To find the case fatality rate, divide the total number of people diagnosed with Covid-19 by the number of people who have died. It is easy. The reason it is probably not accurate is because the total of confirmed Covid-19 cases will not represent all cases.

Some people will have the disease, but never get tested. Perhaps because they have no symptoms, perhaps because they fear paying for a test, or perhaps because no tests are available. There are myriad reasons. The number of people confirmed to have died of Covid-19 is also almost certainly an undercount.

For these reasons, the case fatality rate does not accurately represent an infected persons risk of death from Covid-19, although estimates put it between 3 and 5%, according to Markel.

The death rate often referred to by Trump is the number of deaths in proportion to a population. In other words, it measures the risk of death from Covid-19 across a whole population.

Of the 20 nations currently most affected by Covid-19, the United States has the second-highest death rate at about 42 people killed by Covid-19 per 100,000, according to Johns Hopkins Universitys Covid Resource Center tracking project. Only the United Kingdom is higher, with 67 people per 100,000 killed by Covid-19. Of all countries in the world, the United States has the sixth-highest death rate, behind Belgium, the UK, Spain, Italy, Sweden and France.

But keep in mind, experts view this data skeptically as well.

People in my line of work have to take this data with a grain of salt, said Dr Steven Woolf, a professor in the department of family medicine and population health at Virginia Commonwealth University.

Typically, death rates from specific diseases take years to be issued by the US Centers for Disease Control and Prevention. Now, they are being rushed out in a crisis, and it will probably be a long time before we can make solid apples to apples comparisons.

The short answer is: no. Many experts are worried, and predict a surge of deaths to come, in line with infection and hospitalization rates.

There are a number of reasons why the death rate might not have risen yet, but the most important is that death is a lagging indicator.

If you look at the beginning of the pandemic in the US, it wasnt really until the middle of April when we saw the huge spike in deaths in New York and New Jersey, said Woolf. Im hopeful that the fact that we havent seen the death count start climbing in an exponential way is because were more effective at treatment now, but Im quite worried thats coming.

For other reasons, the death rate may never reach the levels seen in New York City. The average age of Covid-19 diagnosis has dropped since the beginning of the pandemic. That means the outbreak is now being driven by young people who are less likely to die from Covid-19, which could be a factor in the declining death rate.

Second, the surge is spread over a wider geographic area than the one that overwhelmed hospitals in New York City. Mortality may rise as hospitals become overwhelmed and have fewer resources, experts at Johns Hopkins have said. If hospitals become overwhelmed, nurses may be forced to care for a higher number of patients, and care may suffer.

Last, there are unknowns. The coronavirus has been with us since January, or roughly seven months. It is still novel.

The CDC is forecasting up to 170,000 Americans could have died of Covid-19 by 8 August, with the worst-affected states coinciding with the worst outbreaks of disease or where residents have the worst health. Arizona, Florida, Idaho, Montana, Oklahoma, South Carolina, Tennessee, Texas, Utah, the Virgin Islands and West Virginia are expected to bear the most deaths.

We see a pattern where the states ignored public health guidance and opened up too early, said Woolf. The result is that were seeing not only a surge in cases in a public health crisis in those states, but the very economy they were trying to preserve by opening early is now being put in jeopardy.

Even the extraordinary toll Covid-19 has already taken is likely to be a huge undercount. In research published in the Journal of the American Medical Association Network, Woolf and his co-authors reported up to one-third of Covid-19 deaths may be missed in official tallies. Although the study only looked at one eight-week window, if the findings hold true, it could represent tens of thousands of Americans nationally.

You dont really have to be an epidemiologist to look at the numbers on the nightly news and know something is going terribly wrong in the United States, said Woolf. Its not a subtle difference you can wash away with statistical analysis.

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Coronavirus death rate: what it is, what it isn't and why US can expect it to rise - The Guardian