News: COVID viral load peaks later compared to early in the pandemic, study says

Viral load in COVID-19 infection in the Omicron era peaks about four days after the onset of symptoms, according to a new paper published in Clinical Infectious Diseases. This stands in contrast to the peak at symptom onset that was evident early in the pandemic and has driving implications for rapid testing, researchers of the study said. The study’s results have also generated discussion among experts, who believe the shift is likely due to high levels of population immunity, MedPage Today reported.

Nira Pollock, MD, PhD, of Boston Children's Hospital, and colleagues wrote that the findings have "major implications for testing practice going forward."

Researchers studied a cohort of 348 people newly diagnosed with COVID on PCR testing from April 2022 to April 2023, when Omicron was the predominant variant, 91% of whom had a history of infection, vaccination, or both. They found that median cycle threshold (Ct) values hit their lowest point on PCR testing, which is consistent with peak viral load, on the fourth day of symptoms.

Using Ct values to predict rapid antigen results, the researchers estimated a sensitivity of 30% to 60% on the first day of symptoms, rising to 80% to 93% on the fourth day of symptoms.

In comparison, researchers said a single negative antigen test had "reasonable negative predictive value" early in the pandemic, and most studies reported 90% to 95% sensitivity in the first week of symptoms. Overall predicted sensitivity in the first week was about 60% to 80% in this more recent study, however.

“Our data in combination with others' suggest that symptomatic individuals testing positive for SARS-CoV-2 by PCR currently may not reliably test positive on a rapid antigen test until the 3rd, 4th, or even 5th day of symptoms,” the researchers wrote. “Though testing negative on an antigen test suggests that the person is less likely to be viral culture-positive (and presumably infectious) at that moment than someone who tests positive, we note that our data indicate that viral load would be expected to be rising, and not falling in the window after that early false negative test.”

Paul Sax, MD, editor-in-chief of Clinical Infectious Diseases and director of the Infectious Disease Clinic at Brigham and Women's Hospital in Boston, told MedPage Today that the findings suggest people may want to avoid going to work or socializing when they start feeling symptoms before they reach the height of their viral load, and possibly, infectiousness. He said it also calls into question whether a five-day isolation guideline is appropriate, as people may continue to be infectious after day four or five.

Editor’s note: To read MedPage Today’s coverage of this story, click here. To read the full study, click here.