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Why is it necessary to carry out massive diagnoses of SARS-CoV2?

Why is it necessary to carry out massive diagnoses of SARS-CoV2?

Posted by Dr Héctor Rangel, PhD Immunology on 12th May 2020

From the beginning of the epidemic in Wuhan City, China, until its global dispersal that led the World Health Organization to declare the pandemic, more than 3 million people have been infected and around 250 thousand have died due to COVID-19.

Peter Medawar (1960) Nobel Prize in Medicine said that viruses were a bundle of bad news wrapped in protein, given the above figures, it can be affirmed that this is a correct definition.

Person-to-person transmission of SARS-CoV2 occurs when viral airborne particles generated by infected people release droplets of saliva while talking, coughing, or sneezing (Fig. 1). Saliva droplets can be of different sizes, those of "large and medium" sizes quickly fall to nearby surfaces, floors, tables, computers, etc., while those of microscopic sizes can stay in the air for up to 3 hours.


Figure 1: Saliva drops released during sneezing. It should be noted the different sizes and the wide space occupied by the particles.

Most cases of new infections occur from particles suspended in the air or those that were deposited on various surfaces. The route of entry of the virus is the nasal, oral and even ocular mucosa.

An infected person may or may not manifest symptoms, and this has been one of the greatest challenges in combating the global spread of the virus. Hence the need to promote hygiene measures much more frequently, hand washing with soap and water, use of antibacterial gels with 60-70% alcohol, and the use of face masks. Additionally, it is necessary to enhance the disease screening system in order to detect suspicious cases, whether they are symptomatic or not and be able to trace the contacts related and thus apply efficient measures of containment, mobility limitation (quarantine) and symptom monitoring.

The disease detection can be done in two main ways. The first, more efficient and recommended by the World Health Organization, is based on the direct detection of the virus using the PCR technique, "polymerase chain reaction". This technique has an extremely high efficiency, allowing in some cases the detection of at least 10 viral particles in a sample. The sample to be analyzed can be a nasopharyngeal swab, a bronchoalveolar lavage, anal swab, sputum, lung tissue, among others. These infectious samples must be processed in biosafety level 2 environments using a Biosafety Cabinet Class II A2 to subsequently undergo with molecular techniques (PCR) that can be performed in a laboratory with appropriate molecular biology and amplification platform.

The second way of detecting the viral infection is indirectly, measuring the serological response against viral protein, rapid test or ELISA (Enzyme Linked immunological assay) can be used. In these cases, a blood sample is taken from the patient (whole blood or serum) in order to detect the presence of antibodies directed against the virus.

Figure 2. Rapid test for COVID-19. A blood drop is added to the hole on one side of the cassette and a diluent is added. The positive samples are identified by a dark band in the IgM and/or IgG, additionally, the reaction control (C) will also have a dark band.

The efficacy of usage of one detection method or another will depend on the stage of the infection when the sample is taken. Once an infection occurs, the virus attacks the upper airways and begins a replication process in that area. As time passes, the lower airways are affected and the virus tend to become undetectable in the upper airways. Figure 3 shows a description of the course of the infection and shows when detection techniques are useful.


Figure 3 Evolution of infection of SARS-CoV2 and immune response. Importance of diagnostic strategies are labeled in blue (molecular diagnostic) and green (serological diagnostic), both are complementary depending on the time of detection.

It is crucial to implement and apply correctly all the diagnostics tools available for SARS-CoV2 in order to provide the health authorities with the decisive tools needed to combat the high rate of virus transmission from asymptomatic persons or those with a mild flu-like syndrome who put at risk the rest of the population. The identification of positive cases allows the application of containment measures to stop viral dispersion and to control the epidemic, thus avoiding the collapse of the health system.

Further research must be carried out as it is still too early to assume that a person with antibodies is effectively protected against SARS- 2. Most countries have adopted quarantine measurements to reduce the viral transmission. Due to the absence of vaccine and proven treatment, basically the most effective way to control the spread of the virus is to detect positive cases, by using the aforementioned methods and follow up on all related contacts, in this way a low transmission rate can be maintained, which would lead to the eradication of the virus or at least to reduce the number of cases to minimum levels.

#stayhome is the safest option