A lot of noise was made when the American company Moderna announced three weeks ago a candidate for a vaccine against severe acute respiratory syndrome coronavirus, Sars-CoV-2. The expectation created, however, was premature.
It stood out, with justice, the feat of developing a preparation with immunizing potential just 42 days after the genetic sequence of the virus was known. Although reports have reported that the vaccine – if it will work – would only be available in perhaps a year and a half, less attention has been paid to the hypothesis that this deadline makes it harmless.
No one yet knows what will happen to the covid-19 disease. The pandemic can be contained in just over a year, as was the case with the first coronavirus, Sars-CoV-1, in 2003 and 2004.
Sixteen years ago, there was also rush to create vaccines against Sars, as well as in the case of the Middle East Respiratory Syndrome (MERS), which particularly affected Saudi Arabia between 2012 and 2019. Few know that not even one of them has yet been tested and approved, or that the decisive clinical trials will only start in December this year.
Obtaining a license to market a vaccine is not trivial. Under normal conditions of temperature and pressure, it takes about three years, not counting the time of product development. Even with fast tracks open to epidemics and serious pandemics such as Ebola and Sars, it hardly happens in less than 18 months or two years.
This is not about bureaucracy, but about essential precaution. The vaccine announced by Moderna will have its safety tested directly on healthy volunteers, perhaps as early as next month, but its effectiveness is usually verified first with infected guinea pigs, to see if the product is able to stop the installation and multiplication of the virus in the body.
This type of experiment with humans is not done, for obvious ethical reasons. The preferred animal models for this are mice, easy and inexpensive to maintain and reproduce. However, it soon became apparent, when the outbreak was still restricted to China, that these rodents do not become infected with CoV-2.
The solution would be to resort to genetically modified mice, years ago, to exhibit receptors in their cells compatible with the invasive device of CoV-1 and thus become susceptible to SARS. But these strains were discontinued.
They only had frozen sperm samples from the males, which have now started to be used again. However, the animals will only be ready in sufficient quantity there by April or May.
Another factor justifying the utmost caution has an abstruse name, something like antibody-dependent enhancement (“antibody-dependent enhancement”, in English). The paradoxical phenomenon, which is not very common, happens when antibodies produced against the virus end up helping it to penetrate cells or multiply in the defense system's own soldiers, such as macrophages.
It is recommended to establish certainty that this possibility is ruled out for a certain vaccine candidate before starting to apply it around. It is because of this possibility of adverse reactions that tests are carried out first with animals and then with very few humans.
Moderna's early vaccine, which is based on an mRNA platform, has to overcome all these barriers. It turns out that no company vaccine employing this technology has yet been approved by the FDA, the US drug agency.
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