People without symptoms transmit more than half of all cases of the new coronavirus, according to a model developed by researchers from the Centers for Disease Control and Prevention., published in ” JAMA Network Open».
These results reinforce the importance of following the recommendations, regardless of whether or not there are symptoms, such as the use of masks, hand hygiene, social distance and conducting a coronavirus test.
According to this model, 59% of all transmission came from people without symptoms. That includes 35% of new cases from people who infect others before they show symptoms and 24% from people who never develop any symptoms.
“The bottom line is that controlling the covid-19 pandemic is going to require controlling the silent pandemic of transmission from people without symptoms,” says Jay C. Butler, CDC deputy director of infectious diseases and co-author of the study. «Available community mitigation tools should be used widely in order to slow the spread of SARS-CoV-2 from all infected people, at least until we have the vaccines.
The study looked at a variety of presymptomatic transmission scenarios and transmission from infected individuals who never develop symptoms.
In the absence of an effective and widespread use of therapies or vaccines that can shorten or eliminate infectivity, the control of SARS-CoV-2 cannot depend solely on identifying and isolating symptomatic cases; even if implemented effectively, this strategy would be insufficient, the researchers write.
«Measures such as the use of masks and social distancing allow people to protect themselves and, if they are infected, reduce the risk to their communities», They write in the article.
In addition, they add, these measures can be complemented with strategic testing of people who are not ill, such as those who are exposed to known cases (eg, contact tracing) or who are at high risk of exposing other people (eg. Eg, staff of the congregated facilities, people with frequent contact with the public).
The emergence of a more contagious variant, first detected in the UK, further highlights. the importance of those guidelines. “Those findings are now bold, italicized, and underlined,” Butler told The Washington Post.
The model agrees with previous estimates of the contribution of asymptomatic spread.
However, for some experts, such as Muge Cevik, from the University of St. Andrews in Scotland, some of the model’s assumptions are wrong. Cevik said that the best estimate of the relative contagion of people who have no symptoms, compared to those who do, was 35%, according to a review of the scientific literature published in September.
Instead, the study authors estimated, at the start of the study, that people without symptoms were 75% contagious. That figure, Butler says, came from his own analysis of the literature, including peer-reviewed and pre-printed research.
Cevik also pointed out to the Post that the study does not take into account the environment where the spread occurs. “Asymptomatic transmission may be important, but it can be much more important in homes and residences. That could mean we need to do much more specific testing for high-risk populations, ‘as opposed to mass screening.
It is not yet known with certainty if vaccines stop the transmission of the coronavirus and it was not a scenario addressed in this model. «Data on the impact of vaccines on asymptomatic infection are very limited», Indicates Butler.
Clinical trials of mRNA vaccines, authorized in December, concluded that the vaccines are capable of preventing symptomatic diseases. But those trials did not determine whether vaccinated people can spread the pathogen.