COVID: high risk tranmission from household contacts

 A study from Singapore found that household contacts in particular seem to have higher rates of SARS-CoV-2 transmission, frequently asymptomatic (see covid household contacts lancetinfdis2020 in dropbox, or doi.org/10.1016/ S1473-3099(20)30833-1) 

 

Details:

-- retrospective cohort study of all close contacts of confirmed Covid-19 cases, between January and April.

-- 7770 people close contacts were involved (1803 household contacts, 2319 work contacts, and 3588 social contacts), linked to 1114 PCR-confirmed index cases. There were also 2626 transport contacts (eg, in the same car), though these were not considered close contacts

-- median age of contacts 33yo, 52% female; median age of index case 39 yo

 

-- Close contacts:

    -- household contact: those who shared a residence with the index Covid case (26% of household contacts underwent subsequent symptom-based PCR testing)

    -- non-household close contact: at least 30 minutes within 2 meters of the index case (15% of work contacts and 13% of social contacts underwent symptom-based PCR testing)

-- all patients in Singapore with Covid received inpatient treatment, with access restricted to healthcare providers, independent of patients' symptom severity (in order to isolate them from further spread of virus). The median interval between symptom onset and admission to the hospital for the index case was 5 days

-- all close contacts were quarantined for 14 days with symptom monitoring by telephone 3 times a day

-- symptomatic contacts had PCR testing

-- adults who completed quarantine without a positive PCR test were offered serology done at least 2 weeks after their quarantine end date, but only 1150 of 7582 (15%) did so

 

Results:

-- symptom-based PCR testing detected 188 Covid cases, and 7582 close contacts completed quarantine without a positive PCR (only done in those with symptoms)

-- the secondary clinical attack rate was:

    -- 5.9% (4.9- 7.1%) for 1779 household contacts

    -- 1.3% (0.9 - 1.9%) for 2231 work contacts

    -- 1.3% (1.0- 1.7%) for 3508 social contacts

    -- neither sex of the contact nor symptom duration of the index case were statistically associated with secondary clinical attacked rate

    -- those < 30 years old were less likely to be diagnosed with Covid

-- analysis of serologic testing and symptoms found that a symptom-based PCR testing strategy missed 62% of Covid diagnoses, and 36% of individuals with SARS-CoV-2 infection were asymptomatic

    -- but serology tests were only positive for 29 (6%) of the 524 of household contacts, 6 (3%) of 207 work contacts, and 9 (2%) of 419 social contacts

 

-- high transmission rates for household contacts:

     --sharing a bedroom, multivariate odds ratio 5.38 (1.82-15.84), p=0.0023

     -- being spoken to by index case for more than 30 minutes, OR 7.86 (3.86-16.02), p<0.0001

-- high transmission among nonhousehold context:

    -- exposure to more than one covid case, multivariable OR 3.92 (2.07-7.40), p<0.0001

    -- being spoken to by an index case for at least 30 minutes, OR 2.67 (1.21-5.88), p=0.015

    -- sharing a vehicle with index case, OR 3.07 (1.55-6.08), p=0.0013

-- there was no independent association for contacts who had indirect contact, meal sharing, and lavatory co-usage

 

Commentary:

-- the study was based in Singapore, where (unlike the US) there has been extensive contact tracing for every diagnosed Covid-19 case, and mandated legally-enforced quarantine and intensive health surveillance of close contacts. Hence, an excellent locale for a robust study

    -- and, unlike the US, Singapore was prepared for the current SARS-CoV-2 outbreak following the 2003 SARS epidemic, likely contributing to its overall low secondary attack rate among household and nonhousehold contacts

    -- their aggressive identification/isolation of close contacts also helped decrease their downstream continuing infection rates

-- this study therefore adds more granular and rigorous data about contact infection, which could help guide messaging about high vs low risk behaviors

-- they did serologic testing to find missed covid diagnoses in asymptomatic SARS-CoV-2 positive cases, finding that 2/3 were missed by assessing only self-identified symptomatic people by PCR

 

Limitations:

-- as a retrospective study, there may have been recall bias in how well people remembered if they had any potential Covid-related symptoms (which, as we know, can be as little as mild symptoms of a regular old viral URI), though temperatures and symptoms were recorded 3 times a day, which should pick up many people

-- the mathematical modeling from antibody data only included 15% of contacts without a symptomatic Covid diagnosis, and such modeling relies on a limited number of input variables and could potentially not include key ones

-- the use of facemasks was quite low at the time of the study, so their effectiveness could not have been assessed (mask wearing was made mandatory near the end of the study)

-- there was no baseline serology assessed, so conceivably some with positive serology had had viral infection prior to their exposure to the index case above. In the above study they did find that 7 of the 15 symptomatic contacts had negative PCR tests. was this from older infection ? false negative PCRs?

-- And, serology results were positive for only small numbers of patients, limiting the generalizability of the results

 

So, this study reaffirms other less robust studies finding that indoor, close contact is likely to increase the spread of SARS-CoV-2, which brings up a few points:

--it is likely that the stagnant air indoors is largely responsible for the increased concentrations of virus in the air, with less air turbulence than found outside to dilute that concentration. And, this is likely to be worse in the winter, when windows are more likely to be closed, and the air perhaps even more stagnant (and the virus can stay in the air for up to 3 hours)

--it is likely that people do not adequately distance themselves indoors vs outdoors. There tend to be more people in a smaller space indoors so distancing is harder. and many are close family members or friends, so psychologically it is harder to embrace distancing than with strangers outdoors

--we are likely around people indoors for much more extended periods of time than when walking outside. And it does seem likely that the combination of many short close interactions may increase the likelihood of disease transmission. see blog referenced below

--there is even the potential contribution of UV light outside, which seems to kill the virus (though the intensity of this UV light does decrease in the winter). perhaps a role for indoor UV lights????

 

--so, all of this adds up to a potentially really bad scene in the upcoming winter with more people inside for more time…

--which means that the imperative for us clinicians is to emphasize the importance of distancing/masks and limited interactions with others while at home

    -- this is understandably a miserable prospect, given the already severe limitations and bad consequences of the huge disruptions of social relations we have been experiencing, and all likely to get worse in the winter

    -- but we know that a very substantial number of infected people are asymptomatic, hence the need for "universal precautions"

--and, there was a recent Swedish study finding that Covid-19 death rates in those >70yo were higher when they were living with working-aged household members (<65yo) than with older people (>65yo). See covid inc mortality if older and living with young ones Lancet2020or doi.org/10.1016/ S2666-7568(20)30016-7. not surprisingly, younger people are more likely to go out and have higher exposure to the virus. All of this suggesting that we should reinforce social isolation even moreso in housing where younger people live

 

Relevant prior blogs:

-- http://gmodestmedblogs.blogspot.com/2020/08/covid-transmission-and-distancing.html , which reviews the data on appropriate distancing, finding that 6 feet is often not enough (and, I would add, getting much more than 6 feet indoors is likely impossible for many of us in smaller houses/apartments); also some evaluation in the blog on the potential efficacy of UV light

-- http://gmodestmedblogs.blogspot.com/2020/10/covid-infection-by-multiple-very-brief.html , which reviewed a Vermont case suggesting that multiple short exposures, mostly < 1minute, can lead to viral transmission, even to people wearing masks. This study led to a reconsideration/redo of the prior recommendation of less than 6 feet for 30 minutes.

-- http://gmodestmedblogs.blogspot.com/2020/08/covid-severe-psych-substance-use.html , which documents the tremendous increases in psych issues as well as substance use problems associated with covid (again likely to get worse with the winter, for many reasons as above as well as the added layer of seasonal affective disorder, which in its milder form is rampant in northern climes). ??add artificial bright lights to the UV lights???????

geoff

 

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