The Delta variant, officially known as B.1.617.2, emerged as a highly transmissible strain of SARS-CoV-2 that rapidly spread across the globe and became the dominant form of the virus. This variant was characterized by mutations in its spike protein that enhanced its ability to infect human cells and evade some immune responses. The widespread dominance of the Delta variant raised questions about whether the experience of having COVID-19 had changed compared to earlier phases of the pandemic.
Symptoms of Earlier COVID-19 Variants
The initial presentation of COVID-19, caused by the original strain and early variants like Alpha (B.1.1.7), was often defined by a specific set of symptoms. The classic signs included a persistent cough, fever, and significant fatigue. Beyond the respiratory and systemic complaints, a defining characteristic was the high prevalence of chemosensory dysfunction.
Many people infected during the early waves reported a sudden loss of taste (ageusia) or smell (anosmia), which was a strong indicator of COVID-19 infection. While other symptoms like muscle aches, headache, and shortness of breath were common, the loss of smell and taste became a widely recognized hallmark of the infection. Gastrointestinal issues were also noted in the early variants.
The Shift in Delta Variant Symptom Manifestation
The symptom profile underwent a noticeable shift when the Delta variant became the prevailing strain. Instead of the classic triad, infections with the Delta variant often presented more like a severe common cold or flu. The most commonly reported initial symptoms began to include headache, sore throat, and a runny nose.
Fever remained a frequent symptom, but the overall presentation was more concentrated in the upper respiratory tract. Conversely, the defining symptom of the early pandemic—the loss of taste and smell—became significantly less frequent with the Delta variant. While anosmia and ageusia were still possible, they were not reported in the top ten most common symptoms in some studies during the Delta wave. This change meant that people were more likely to mistake their infection for a seasonal illness, challenging early detection and isolation.
Differences in Disease Progression and Viral Load
Beyond the initial symptoms, the Delta variant exhibited changes in how the disease progressed. The Delta variant was associated with significantly higher viral loads in the respiratory tract compared to the original ancestral strain. Research suggested that the viral load could be up to 1,000 times higher in the first positive test, which contributed to its increased transmissibility.
The time from exposure to the onset of symptoms, known as the incubation period, also appeared to be shorter with the Delta variant, sometimes reported as four days compared to six days for the ancestral strain. Although initial symptoms could mimic a cold, the potential for rapid progression to severe illness was increased. The Delta variant was linked to a higher risk of hospitalization and was associated with a greater chance of requiring oxygen or developing pneumonia compared to earlier variants.