Delayed Peak Viral Loads in COVID-19 and Implications for Testing

A study published in Clinical Infectious Diseases provides valuable insights into the evolving patterns of SARS-CoV-2 viral loads, particularly in the context of a highly immune population. The study, titled “The New Normal: Delayed Peak Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) Viral Loads Relative to Symptom Onset and Implications for Coronavirus Disease 2019 (COVID-19) Testing Programs,” highlights significant findings for COVID-19 testing and management strategies.

Key Findings of the Study:

  1. Participant Demographics: The study involved 348 newly-diagnosed PCR-positive individuals for SARS-CoV-2, predominantly women (65.5%) with a median age of 39.2 years. A significant proportion (91.1%) had a history of vaccination, natural infection, or both.
  2. Viral Load Dynamics: Contrary to early pandemic observations where viral loads peaked at symptom onset, this study found that in a highly immune population, median SARS-CoV-2 viral loads peaked around the fourth or fifth day of symptoms. For influenza A, viral loads peaked sooner, typically on the second day of symptoms.
  3. Implications for Testing: The delayed peak in viral loads has implications for the effectiveness of antigen rapid diagnostic tests (Ag RDTs). The study estimates that Ag RDT sensitivity was between 30.0% to 60.0% on the first day of symptoms, increasing to 59.2%–74.8% by the third day, and reaching 80.0%–93.3% on the fourth day. This contrasts with earlier pandemic phases where rapid antigen tests had higher sensitivity within the first week of symptoms.
  4. Changing Testing Practices: The findings suggest that the current practice of using a single rapid antigen test early in the symptom course may not be as effective in a population with a high level of immunity, especially post the early 2022 Omicron variant surge.
  5. Study Limitations: The study acknowledges limitations, including the use of PCR cycle threshold (Ct) values as a proxy for viral loads, potential non-generalizability to all populations, and limited data on the impact of varying forms of immunity on viral load trends. However, the researchers believe the findings are likely generalizable to the larger U.S. population at this pandemic stage.

Conclusion: The study underscores the evolving nature of SARS-CoV-2 infection dynamics, particularly in populations with widespread immunity. It highlights the need for updated testing strategies that account for delayed peak viral loads. This adjustment is crucial for accurate COVID-19 diagnosis, especially considering the different patterns observed with influenza A. The study emphasizes the importance of ongoing monitoring and adaptation of testing practices in response to the changing characteristics of the virus​​​​.

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