Editor's Note: (Kent Sepkowitz is a CNN medical analyst and a physician and infection control expert at Memorial Sloan Kettering Cancer Center in New York. The views expressed in this commentary are his own. View more opinion at CNN.)
(CNN) For almost two weeks, three countries have led the rest of the world in the daily number of new Covid-19 infections: The United States, Russia and Brazil.
This makes some sense -- each is in the top 10 of most populous countries with well over 125 million people and each has a few very crowded cities. Furthermore, each country was disorganized and disbelieving as the disease settled in their region.
But one important difference separates Russia from the other two nations. As of Monday, in the United States, which has been in the midst of the pandemic for almost two months, the mortality rate from Covid-19 was 6%, according to Johns Hopkins; in Brazil, hard-hit for a month, the mortality was nearly 7%. And in parts of Western Europe, including Italy, France, Spain, Belgium and the UK the mortality rate was over 10%.
But in Russia, where the pandemic arrived about the same time as Brazil, the mortality rate sat at less than 1%.
How can this be?
Dr. Elena Malinnikova, the chief of infectious diseases in the Russian Ministry of Health, has a simple explanation: The low mortality is due to timely detection of infection as well as the fact that that Russians tend to see their doctor soon after symptoms appear.
Russian journalists have reported that more than 60% of all cases diagnosed in the country have been in Moscow, which has a younger and healthier population than rural areas.
No matter the reason, the extremely low mortality rate is quite unexpected. So, I compared various factors to try to understand what's going on.
The testing difference between Russia and Brazil is enormous. Dr. Malinnikova and Russian leaders should be very proud. At the time of writing, they have tested 40,995 people per million population. In contrast, Brazil has a very low rate, at 3,459 tests per million citizens. (for comparison, the US has a rate of 30,937 per million).
As we have seen in Switzerland, Germany and other countries with extensive testing, mortality rates start low because so many cases are identified and never progress to serious illness though as the German epidemic has matured, the fatality rate has increased from less than 0.5% to about 4.5%.
The profound testing difference may indeed be the main reason, but other factors can contribute to risk of death from Covid-19 infection, including being a male, advanced age, heart disease, chronic lung disease, diabetes and obesity, for which comparative countrywide information is available.
Of note, Russia has high rates of these comorbid conditions, especially among men.
Unfortunately, neither Russia nor Brazil has released much information regarding the age and sex distribution of the cases or the deaths in their country, though in late April, Brazil reported that 59% of coronavirus deaths were in men. However, per the World Bank, 15% of Russia's population is 65 years old or older, compared to Brazil with 9% (the US sits at 16%). This difference would predict that Brazil would have a lower mortality rate than Russia.
Cardiac disease also is much more prevalent in Russia than Brazil. A recent study suggests that by any measurement, Russia has at least twice the rate of complications from cardiovascular disease as Brazil -- another reason Covid-19 death rates would be lower, not higher, in Brazil than Russia.
No statistics for rates of chronic lung disease by country are available. However, smoking, a major cause of lung disease, is more common in Russia (57% of men and 23% of women) than Brazil (17% of men and 10% of women). In contrast, death from lung disease is lower in Russia (14.5 per 100,000) than in Brazil (26.6 per 100,000).
Diabetes is about half as common in Russia (6%) compared to Brazil (10%) while rates of obesity are about the same: As of 2016, Russia was 70th in the world with a 23% prevalence while Brazil sat at 22% for 82nd in the world. (By comparison, the United States had the 12th highest rate at 36%).
Therefore, the known and measurable risk factors for severe disease point to Russia having at least a comparable, if not a higher, death rate than Brazil.
So, is Dr. Malinnikova correct? Is the difference really due to testing and the apparently efficient Russian health care system?
Perhaps, but there may be other ways to explain the differences. We do not know, for example, how Russia attributes death to Covid-19. If a person with heart disease dies with the infection, which condition is the "cause?" How quickly does information come to Moscow from the rural areas which may have higher death rates given the higher rate of comorbidities?
And what about nursing home deaths -- are these being included as Covid-19 related, even if a test was not performed?
As in the US and elsewhere, accurate classification of a Covid-19-related death remains very important to our understanding of the disease and the effectiveness of our attempts to control it. However, certainty about how, exactly, a person died remains a very difficult determination.
Diseases are not independent of each other: someone with heart disease will predictably fare worse with pneumonia than someone without heart disease -- but should the person die, what is the true cause of death? Probably a collision of illnesses that collectively overwhelm a person rather than a single nameable cause.
This means that the cause of death can be tilted one way or another while still remaining accurate. And with a disease such as Covid-19, where political tensions are evident in many countries, this latitude (and the temptation to exploit it) calls to mind the chilling statement popularly attributed to Josef Stalin, though it's far from clear that he said it: "It is not who votes but who counts the votes that matters."
In other words, the death rate from Covid-19 in Russia and worldwide is defined not by an internationally agreed upon definition, but by the authorities who are reporting. Once again, we may find our understanding of Covid-19 stymied by an altogether new uncertainty -- this one not medical at all, but entirely the product of political calculations.