Why does Mexico have so many more cases of coronavirus than its Government reports?

Mexico has 3,442 cases of Covid-19 confirmed with diagnostic tests and 194 deaths. However, the authorities estimate that the actual number of patients is eight times higher, up to 28,000 cases. How do you explain this difference?

Faced with criticism that Mexico is one of the countries in the world that performs the least tests, the undersecretary of Prevention and Health Promotion, Hugo López-Gatell, maintains that the country applies a system called "sentinel model" that allows tracking the disease without the need for massive population tests.

Also, and in the face of controversy, this Thursday the Government announced that around 300,000 Covid-19 detection tests will be acquired and will be distributed among the states. This without renouncing the sentinel model.

What is the sentinel model?

This epidemiological surveillance system, used in the country since 2006 for the control of influenza, seeks to optimize resources, so that a segment of the population is tested and, subsequently, the actual extent of the disease is calculated.

"It is a model that admits that not all patients can be found and it has to be adjusted to determine what the true number is in the population," explained Dr. Alejandro Macías, former commissioner for influenza care in Mexico.

In the country, there are 375 units in hospitals and health centers that perform the Covid-19 test on those with severe symptoms and who serve as a sample as if it were a survey.

Based on studies on respiratory disorders throughout the country, the Ministry of Health estimates that the number of patients is 8.2 times greater than the cases detected in these tests, although some researchers believe that the actual number may be 10 or even 20 times greater.

Is this system effective?

Undersecretary López-Gatell maintains that in an epidemic "it is not feasible" to follow up on all patients and stresses that it is more relevant to know how the disease spreads and what groups it affects instead of the total number of cases.

According to Macías, sentinel surveillance is a "useful" model for the control of better-known diseases, such as influenza, but in the case of Covid-19, "international evidence says that the countries that have done the most tests and earliest fared better, "like South Korea.

"The authority says it does not want to do a lot of testing, that it is not useful. But it goes against international evidence," he said. The researcher warned that it is unknown who are the thousands of patients who have not undergone the coronavirus test and therefore "are in the community transmitting the disease."

Along the same lines, epidemiologist Rogelio Navarrete added that the objective of the doctors should be to identify SARS-Cov2 and that "the orthodox is that every person who meets criteria should be screened and kept in isolation." "That is expensive for the ministries of health and what they do is optimize resources," he said of the sentinel model.

Why such a big difference with the US?

The difference in the way of accounting for cases is a factor that explains why Mexico has far fewer confirmed patients than the United States, where the number of confirmed Covid-19 infections is close to half a million.

"If more diagnostic tests are performed in one place, there will be more numbers of cases compared to the place where only a certain proportion of tests are performed and not all suspicious or asymptomatic cases," Navarrete evidenced.

In addition, he pointed out that there are many other factors such as the type of population, people's response to social distancing measures or the health resources of each country.

In this sense, Macías stressed that the United States "has much more migration and international mobility" than Mexico, so a city as cosmopolitan as New York is precisely the main focus of the disease in the country.

How long will it last in Mexico?

The peak of the pandemic, which reached the country in late February and that the Government has faced with the closure of most economic activities in April and exhorting the population to stay at home, is not yet in sight.

"We have not seen anything in Mexico yet," said Macías, who explained that the contagion curve is still in an acceleration phase, to then stabilize and begin to decrease in a process that will last at least two months.

"In Mexico there is a 'light' distancing, nobody is forced. With a greater intensity of the epidemic, there will probably be more actions. The problem is that the results of the social distancing are not seen until 15 days," he said.

For Navarrete, the best way to combat the disease is with immunity, which is why "the most prominent institutes in the development of vaccines must have resources and work hard." "If we don't get the disease to enter more slowly, Mexico will be as bad or worse than Italy. We will find out in late April or early May," Macías concluded.

According to the criteria of The Trust Project

• Mexico
• U.S
• Infectious diseases
• Coronavirus

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