Lethality, positivity and COVID-19 infections: this is how the low number of tests affects official statistics in Mexico

(Photo: Rogelio Morales / Cuartoscuro)

Since the COVID-19 pandemic reached Mexico, the national strategy to deal with it – led by the Undersecretary of Health, Dr. Hugo López-Gatell – has refused to give priority to testing. He has expressly said so repeatedly. Rather it was prioritized mitigate cases (lessen the scourge of the virus) and even the famous concept of “herd immunity” was brought to the table (at least initially).

Thus, within a year of the first positive case registered in Mexico, that country is among those with the least COVID-19 tests on its inhabitants, at least in the American continent. For example, the real-time statistics site Worldometer records that in the Aztec country 37,782 tests are carried out per million inhabitants, that is, only 3.77% of the population is tested.

This Tuesday, in his usual morning conference, the president Andrés Manuel López Obrador celebrated that, according to the number of infections recorded by the health authorities, the level of contagions is being reduced at the national level.

Right there, Dr. López-Gatell explained: “24 of 32 states have at least two weeks of reduction in the intensity of the epidemic, reduction of both new cases, deaths and hospitalization”.

On a first impression it would seem that such sayings could be far from reality, being a country of 126 million inhabitants, of which those who have been tested for COVID-19 do not represent even 4%, or that of that number of tests, at least half is positive.

(Photo: EFE / José Pazos)

(Photo: EFE / José Pazos)

However, for the doctor of mathematical sciences, Arturo Erdely, as regards the virus behavior in Mexico, the number of tests does not drastically affect, since for know if the level of cotagios is really going up or down, he points to a key aspect: the percentage of positivity.

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“If positivity is going down hand in hand with infections and tests, then yes”, said to Infobae the statistician. That is to say, if only the number of tests were lowering but not the level of contagion, there would be more cases. “And it’s not what we’ve been seeing for the last few weeks; positivity has been declining along with infections and tests applied. This is one sign that there is indeed a decline in the number of infections, at least in the official figure ”.

So, according to the dynamics of the federal government, in January –for example– the number of tests and the number of positive cases increased because increased the number of people experiencing symptoms. Now contagions and tests are going down because fewer people are currently showing signs of the virus, that is to say, the infections are going down.

(Photo: EFE / Jaime Ávalos)

(Photo: EFE / Jaime Ávalos)

But the analyst insisted that the level of contagion can only be calculated based on the figures reported by the government. “Real positivity we have no way of measuring itIt’s all based on official data ”.

What does affect the number of tests

During the pandemic, Dr. Arturo Erdely has made (and publicly shared) his own graphs, based on official figures, to analyze the behavior of the virus in Mexico from the different variables. Hence also the fact that he is so familiar with the subject. And from his follow-up he points out that what it does affect considerably the low amount of evidence of Covid that are made in the country is in the positivity and case fatality rates.

“That is why the positivity rate in Mexico is one of the highest in the world: not because there are much more infections than in the rest of the world, but if the tests are reserved for the cases that look more serious, obviously it is going to have a higher percentage of positive cases. In contrast to countries that do massive tests, since they register much lower rates of positivity ”.

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(Photo: REUTERS / Edgard Garrido)

(Photo: REUTERS / Edgard Garrido)

So we have, for example, in the registry kept by Johns Hopkins University, Mexico appears with the highest level of lethality due to the virus worldwide. But that does not mean that it is the country where COVID-19 is deadliest. If not, because Mexico limits its tests to cases with symptoms and severity, mild or asymptomatic infections are not taken into account in the figure with which the level of lethality is calculated.

“The limited number of tests applied in Mexico causes artificially high levels of positivity and lethality, which make them not comparable with other countries”stressed Dr. Erdely.

The expert exemplifies this with the death rate from COVID-19 in Mexico. “(There) it plays artificially in favor: as few tests are applied, few cases are detected, of those few die, then a few deaths are divided by the size of the population. So just as the fatality rate is artificially high, the mortality rate is artificially low”.


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