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It is not entirely clear to researchers how long one is actually contagious
by Nicola Barone
It is not entirely clear to researchers how long one is actually contagious
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On average, it takes a 30-day period for the coronavirus from when the first positive swab is cleared from the body, which means that half of patients remain positive about a month after diagnosis. On the other hand, 36 days are needed for the disappearance of the manifestation of the first symptoms. This is the evidence that comes from an Italian study in the BMJ Open magazine, carried out by the local health unit-Irccs of Reggio Emilia on a large sample of symptomatic individuals who tested positive for the virus between February and April.
WHO Recommended Practice
The extent of the period in which the subject remains contagious from the time of diagnosis is not yet clear. In fact, a positive swab does not necessarily indicate that the subject is still contagious (thus that there is still an active virus in the body), but simply indicates the presence of viral genetic material in the body. The World Health Organization recommends 13 days of isolation from the onset of Covid-19 symptoms and 10 from the date of the positive swab in an asymptomatic person. “However in Italy – explained to theAnsa one of the study authors, Francesco Venturelli from the Epidemiology Service of the Emilian healthcare company – we adopted a stricter protocol that foresees the need to obtain two negative swabs consecutively to get out of isolation. From the results of our study – continues the epidemiologist – it appears that approximately half of symptomatic patients remain positive 30 days after the first swab “and that, therefore, it is of little use to repeat the swab for 2 or 3 weeks. after diagnosis.
The danger of “false negatives”
Epidemiologists monitored an initial group of 4,538 residents in the province of Reggio Emilia (positive and symptomatic) and found that performing a second swab 2 or 3 weeks after the first one in most cases returns positive. Experts have also found that among those who test negative on a swab performed within the first 3 weeks of diagnosis, the risk of a “false negative” is high, meaning that the swab falsely indicates that there is no virus in the body (result negative test negated to next buffer). In fact, in the observed sample it emerged that the false negative rate is relatively high (1 case out of 5) in the first part of convalescence, potentially putting the subject at risk of ending isolation while remaining positive. This risk is extremely low in the Italian case where, in fact, two consecutive negative smears are required to return to the community.
What indications of the study
However, in other countries a false negative could favor unconscious transmission of the virus. “To reduce the number of false negatives and the number of checks required – the researchers conclude – it might be possible to start testing 4 weeks after the first swab; in short, postponing the first check in symptomatic individuals in isolation could improve the efficiency and security of the protocols “.
