The CDC is just cranking out reports. So here is one on an outbreak in Wisconsin at a summer retreat facility. (CDC Report) A student tested negative a week before the camp. The student then developed symptoms at camp. Eventually 118 out of 152 people were positive. There had been 24 people with evidence of prior infection and none of them tested positive, so at least there is evidence of immunological protection. But the spread among staff and students was obviously swift and comprehensive, with only ten people who could have been infected, not testing positive. These were high school students and they were housed in dormitories. All illnesses were asymptomatic or mild, no hospitalizations. CDC blamed the outbreak on failure to follow pre-camp quarantine recommendations.
The next one comes from my home state and covers infections in health care workers. (CDC Report) In the early phase of the epidemic, up to mid-July, about 21,400 potential health care worker exposures were reported, 5374 of which were classified as high risk. Of the high risk exposures, 81% were in the health care setting and the remainder at home or in social settings. In the period within 14 days after the exposure, 31% of the workers reported symptoms but only half of those had a positive test. The workers or the patients/residents were almost always wearing a mask or other personal protective equipment.
The third CDC report relates to a household transmission study conducted in Tennessee and Wisconsin. (CDC Report) There were 101 index patients in the households. Among 191 contacts, 102 ended up with a positive test in the next 14 days. The index patient was a child in only 14 of the households. Younger children appeared more likely to transmit that older children. Almost all transmission occurred within five days. One-third of the secondary cases were asymptomatic. Some of the supposed secondary transmission could have originated outside the house or occurred at the same time or before the supposed index case.
It is apparent that there is some geographic pattern in the intensity of the epidemic and that this is modulated by population density. We can pretty clearly see these in case resurgences in the US and Europe, much of which is occurring in areas without extensive infection rates earlier in the year. This paper notes that temperature does not appear to be the intermediate factor, but latitude does, suggesting that duration and intensity of sunlight may be the primary factor. Sunlight can inactivate the virus and may raise population vitamin D levels. (Medrxiv Paper) The study looked at case trends in Europe between September 20 and October 18 and compared it to the two weeks preceding temperatures and latitude. There was basically no correlation with temperature, but latitude explained around 75% of the change.
This paper compared upper respiratory tract viral loads in children who were symptomatic versus those who were asymptomatic. (JCM Article) Cycle number from test results was used and converted to viral load estimates. 118 asymptomatic and 197 symptomatic children were the subjects. The symptomatic children had median test results were ten cycle numbers lower (meaning the viral load was higher) than did the asymptomatic children. That is a big difference in viral load and infectiousness.
This study assessed CV-19 prevalence among children in health system day care centers used for health care workers. (Medrxiv Paper) The researchers found that children did not appear to contribute to transmission to parents or the day care workers.
I am not a fan of large scale testing of asymptomatic people, where the risk of false and low positives is great. This paper discusses the drawbacks of such testing. (RS Paper) The rationale is usually that you can identify persons who may become symptomatic and need treatment and more importantly, you can identify potential spreaders who should isolate. The downsides are use of potentially scarce testing kits and reagents, use of staff time, increase in turnaround times for results. You are potentially forcing large numbers of people to isolate, most of whom pose no threat of infecting others. It can cause economic and educational dislocation.