The use of systematic reviews and meta-analyses is of particular interest in a public health crisis, where research is sometimes very limited in sample size, and reproducibility of studies is sometimes difficult.

In this review they find that asymptomatic people are less likely to be contagious than people with symptoms, which may be consistent with other studies.

However, the results of this review and meta-analysis should be interpreted with extreme caution, as the included studies are very different from each other because:

(1) They use very different definitions of what an asymptomatic is.

(2) An asymptomatic person at the beginning of the pandemic (due to test availability) is not the same as an asymptomatic person in the later stages of the pandemic with a high proportion of the population vaccinated. This fact makes the estimates made in the meta-analysis difficult to interpret. 

This work consists of a systematic review of published works, following all the standards for this type of study. It aims to answer two specific questions:

(1) How many people are completely asymptomatic after SARS-CoV-2 infection?

(2) Since it has been reported that viral load can be as high in an asymptomatic person as in a symptomatic person, is the risk of transmission to others the same?

The authors draw two conclusions:

(1) The rate of true asymptomatics may be between 14-50%.

(2) Asymptomatic people have a lower risk of transmission than pre-symptomatic or symptomatic people.

These data confirm observations already made by other studies. As the authors themselves point out, one of the limitations of this work is that it is based on data published up to July 2021, which means that, for example, it does not assess whether the new variants (delta, omicron) change these conclusions. Nor is there a review of how these data change if we include vaccination as a factor that can certainly change the conclusions. 

In any case, if we have learned anything from this virus, it is that transmission from asymptomatic and presymptomatic people is one of the decisive factors that has contributed to the global spread of this virus and to the pandemic situation we are still in. The data from this study may be useful for public health services to implement pharmacological and non-pharmacological measures to curb the transmission of the virus in certain contexts.

The study published in PloS Medicine gives some more robust clues regarding asymptomatic SARS-CoV-2 infections. Based on the pooled analysis of 130 studies, it confirms that purely asymptomatic infections are less frequent than previously thought, and, above all, that infectivity from those infected without symptoms is much lower than from those with clinical symptoms (however mild).