The Corner

Science & Tech

JAMA Study: Asymptomatic COVID Spread Risk Low, Symptom-Onset Risk High

A new meta-analysis of COVID spread — which looked at more than 54 relevant studies with 77, 758 participants — was just published in JAMA Network (published Journal of the American Medical Association). Some on Twitter are touting the study as finding a low risk of asymptomatic transmission.

The study does so find, but commentators may be making too much of that conclusion. The study also finds that there is risk of presymptomatic transmission, and that peak transmissibility of COVID may be at or just prior to symptom onset. From, “Household Transmission of SARS-CoV-2”:

We found significantly higher secondary attack rates from symptomatic index cases than asymptomatic or presymptomatic index cases, although less data were available on the latter. The lack of substantial transmission from observed asymptomatic index cases is notable. However, presymptomatic transmission does occur, with some studies reporting the timing of peak infectiousness at approximately the period of symptom onset.

So, how does one know whether a particular individual is asymptomatic infected (lowest chance of spread), presymptomatic infected (moderate chance of spread), onset of initial symptoms (highest risk), or uninfected (no chance of spread) other than by undergoing continual testing?

In any event, household transmission remains a prime venue for spreading the disease:

Controlling for differences across studies, secondary attack rates were higher in households from symptomatic index cases than asymptomatic index cases, to adult contacts than to child contacts, to spouses than to other family contacts, and in households with 1 contact than households with 3 or more contacts.

The more contacts, the greater the likelihood of infection:

Contact frequency with the index case was associated with higher odds of infection, specifically at least 5 contacts during 2 days before the index case was confirmed,70 at least 4 contacts and 1 to 3 contacts,63 or frequent contact within 1 meter.

The authors suggest that reducing COVID could require isolating the symptomatic infected individual outside the home:

The findings of this study suggest that households are and will continue to be important venues for transmission, even where community transmission is reduced. Prevention strategies, such as increased mask-wearing at home, improved ventilation, voluntary isolation at external facilities, and targeted antiviral prophylaxis, should be further explored.

None of this is terribly surprising. But why some governors are targeting outdoor activities for restrictions when the primary threat clearly involves close, indoor repeated contacts, is beyond me.