Infectivity




Infectivity is indicated by the basic reproduction number (R0, pronounced "R naught") of the disease. SARS-CoV-2 is estimated to have an R0 of 2.2 to 2.5. This means that in a population where all individuals are susceptible to infection, each infected person is expected to infect 2.2 to 2.5 others in the absence of interventions. R0 can vary according factors such as geography, population demographics and density. In New York state R0 was estimated to be 3.4 to 3.8.

On average, an infected person begins showing symptoms five days after infection (the "incubation period") and can infect others beginning two to three days before that. One study reported that 44% of viral transmissions occur within this period. According to the CDC, a significant number of infected people who never show symptoms are nevertheless contagious. In vitro studies have not found replication-competent virus after 9 days from infection. The statistically estimated likelihood of recovering replication-competent virus approaches zero by 10 days.

Infectious virus has not been cultured from urine or reliably cultured from feces; these potential sources pose minimal if any risk of transmitting infection and any risk can be sufficiently mitigated by good hand hygiene.

Patterns and duration of illness and infectivity have not been fully described. However, available data indicate that SARS-CoV-2 RNA shedding in upper respiratory specimens declines after symptom onset. At 10 days recovery of replication-competent virus in viral culture (as a proxy of the presence of infectious virus) approaches zero. Although patients may produce PCR-positive specimens for up to six weeks, it remains unknown whether these samples hold infectious virus. After clinical recovery, many patients do not continue to shed. Among recovered patients with detectable RNA in upper respiratory specimens, concentrations after three days are generally below levels where virus has been reliably cultured. These data were generated from adults across a variety of age groups and with varying severity of illness. Data from children and infants were not available.

Comments

Post a Comment

Popular posts from this blog

Testing protocols

Image
Drive-through testing In drive-through testing, the person undergoing testing remains in a vehicle while a healthcare professional approaches the vehicle and obtains a sample, all while taking appropriate precautions such as wearing personal protective equipment (PPE). Drive-through centers helped South Korea accelerate its testing program. Home collection In Hong Kong test subjects can stay home and receive a specimen tube. They spit into it, return it and later get the result. Pooled testing In Israel, researchers at Technion and Rambam Hospital developed a method for testing samples from 64 patients simultaneously, by pooling the samples and only testing further if the combined sample was positive. Pool testing was then adopted in Israel, Germany, Ghana South Korea, Nebraska, China and the Indian states of Uttar Pradesh, West Bengal, Punjab, Chhattisgarh and Maharashtra. Open source, multiplexed designs released by Origami Assays can test as many as 1122 patient samples using only 9
Read more

Testing statistics by country

Testing strategies vary by country and over time, with some countries testing very widely, while others have at times focused narrowly on only testing the seriously ill. The country that tests only people showing symptoms will have a higher figure for "% (Confirmed cases as percentage of tested samples or tested cases)" than the country that also tests others. If two countries are alike in every respect, including which people they test, the one that tests more people will have a higher "Confirmed / million people". Studies have also found that countries that test more, relative to the number of deaths, have lower estimated case fatality rates and younger age distributions of cases.
Read more

Methods

Image
Positive viral tests indicate a current infection, while positive antibody tests indicate a prior infection. Other techniques include a CT scan, checking for elevated body temperature, checking for low blood oxygen level, and the deployment of detection dogs at airports. Detection of the virus Reverse transcription polymerase chain reaction Polymerase chain reaction (PCR) is a process that amplifies (replicates) a small, well-defined segment of DNA many hundreds of thousands of times, creating enough of it for analysis. Test samples are treated with certain chemicals that allow DNA to be extracted. Reverse transcription converts RNA into DNA. Reverse transcription polymerase chain reaction (RT-PCR) first uses reverse transcription to obtain DNA, followed by PCR to amplify that DNA, creating enough to be analyzed. RT-PCR can thereby detect SARS-CoV-2, which contains only RNA. The RT-PCR process generally requires a few hours. Real-time PCR (qPCR) provides advantages including automatio
Read more