Tag: viral load

Categories : COVIDTags :

Omicron Viral Load Shedding May Be Unaffected by Vaccination

On By ModernMedia
SARS-CoV-2 virus
SARS-CoV-2 virus. Source: Fusion Medical Animation on Unsplash

A small study published in the New England Journal of Medicine has found that viral load shedding of the omicron variant is similar to other strains, and is not significantly affected by vaccination status.

The SARS-CoV-2 omicron variant has a shorter incubation period and a higher transmission rate than previous variants. Recently, the Centers for Disease Control and Prevention recommended shortening the strict isolation period for infected persons from 10 days to 5 days after symptom onset or initial positive test, followed by 5 days of masking. However, the viral delay kinetics and load shedding of omicron is still unclear.

Using nasal swabs to measure viral load, sequencing, and viral culture, they enrolled 66 participants, including 32 with delta variant and 34 with omicron. Participants who received COVID–specific therapies were excluded; only one participant was asymptomatic.

The characteristics of the participants were similar in the two variant groups except that more participants with omicron infection had received a booster vaccine than had those with delta infection (35% vs 3%). After adjustments for age, sex, and vaccination status, the number of days from an initial positive polymerase-chain-reaction (PCR) assay to a negative PCR assay and the number of days from an initial positive PCR assay to culture conversion were similar in the two variant groups.

The median time from the initial positive PCR assay to culture conversion was 4 days in the delta group and 5 days in the omicron group; the median time from symptom onset or the initial positive PCR assay, whichever was earlier, to culture conversion was 6 days and 8 days, respectively. There were no appreciable between-group differences in the time to PCR conversion or culture conversion according to vaccination status, although the sample size was quite small, which led to imprecision in the estimates.

In these participants with nonsevere COVID, the viral decay kinetics were similar with omicron infection and delta infection. No large differences in the median duration of viral shedding was seen among participants who were unvaccinated, vaccinated but not boosted, and those who were vaccinated and boosted.

Discussing limitations, the authors cautioned that the small sample size limits precision, and there are possible residual confounding variables. Further studies are need to properly correlate culture positivity with infectivity.

They conclude by saying: “Our data suggest that some persons who are infected with the omicron and delta SARS-CoV-2 variants shed culturable virus more than 5 days after symptom onset or an initial positive test.”

Categories : COVIDTags :

Viral Load Alone not Indicative of COVID Transmission Risk

On By ModernMedia
SARS-CoV-2 viruses (yellow) infecting a human cell. Credit: NIH

Viral load as determined by cycle threshold (Ct) has limited utility in guiding decisions regarding isolation and quarantine of COVID patients, according to a study of COVID cases in university students.

Though some in vitro studies indicate that virus load levels in infected individuals affects the successful rate of virus transmission, whether the viral load carried at the individual level can determine transmissibility was unknown. In this study published in The Journal of Molecular Diagnostics, university students underwent regular testing and contact tracing after positive tests, and significant overlap in cycle thresholds (Ct) was found between spreaders and nonspreaders. This brings into question using Ct values to determine transmission rates, with even those with low viral loads able to transmit the virus.
Real-time RT-PCR Ct values represent the number of amplification cycles required for the target gene to exceed a threshold level. Ct values are therefore inversely related to viral load and can provide an indirect method of quantifying the copy number of viral RNA in the sample; however, the use of Ct values as a proxy of viral load is influenced by the assay itself (correlation would stand in the linear dynamic range of the specific RT-PCR assay used) and factors within the sample matrix that can affect amplification efficiency

“We wanted to find whether there was a scientifically sound way to quickly triage students with potential high-risk exposure to COVID positive students for quarantine,” explained co-lead authors Patrice Delafontaine, MD, Department of Medicine, and Xiao-Ming Yin, MD, PhD, Departments of Pathology and Laboratory Medicine, Tulane University School of Medicine. “Some studies have found that the Ct value of the RT-PCR assay is a surrogate for infectivity, and cutoff Ct values have been proposed as a way to guide isolation practices. Through testing and contact tracing, we found that Ct value could not predict transmissibility. We should not overlook positive patients with low viral load, and all positive patients should be quarantined.”

A high-throughput SARS-CoV-2 surveillance testing program was established at Tulane University to support isolation and contact tracing efforts at the campus. Students were tested twice weekly and asked about symptoms they may be experiencing. Contact tracers spoke to all positive case subjects to identify close contacts.
The study looked at 7440 patients who were screened between September 1, 2020 and October 31, 2020, among whom 602 positive cases were identified. From this group, 195 index cases were identified with one or more reported close contacts, who were then tested during their mandated 14-day quarantine period for evidence of transmission from the associated index cases. Of these index cases, 48.2% had at least one contact who became COVID positive, whereas 51.8% of the index cases were nonspreaders with no contacts who subsequently tested positive. Mean Ct values of the spreaders and the nonspreaders were nearly identical.

The researchers then reversed approach, where index cases were traced for 481 students undergoing quarantine due to known exposure to the disease. Eighteen percent of the students became positive during their quarantine. Index cases for the 481 quarantined students were considered spreaders if they were linked to one or more quarantine students with a positive test result, or nonspreaders if they were associated only with students with negative test results. Mean Ct values of the spreader and the nonspreader groups were similar.

The researchers next identified and evaluated 375 positive cases to assess the relationship between symptom presentation and Ct values. Reported symptoms included lethargy, fever, headache, cough, runny nose and gastrointestinal symptoms. Mean and median Ct values were lower in symptomatic cases than in asymptomatic cases, indicating a higher viral load, This suggests that infections with a higher viral load could more often lead to symptom development, or that symptomatic individuals tend to have higher viral loads or maintain their viral loads for a longer period of time. Ct levels may be useful at a population level, in association with symptomatic presentation, to indicate the likelihood of transmission. These values may thus have epidemiologic or surveillance importance.

“Taken together, these index cases suggest that Ct values alone do not predict transmission risk and reporting of Ct values at the individual level, such as by setting a cutoff value of 32, would provide little diagnostic value for case management,” note Dr. Delafontaine and Dr. Yin. “A sensitive and robust SARS-CoV-2 diagnostic testing method is needed to effectively control viral transmission by maximizing the ability to identify and quarantine even those with a low level of virus.”

Source: Elsevier