A fever is an elevated body temperature, commonly defined as 100.4°F (38°C) or higher. While normal body temperature varies, typically between 97.5°F and 98.9°F (36.4°C to 37.2°C), a fever is a temporary internal state, distinct from external factors like hot weather or exercise.
How the Body Generates a Fever
The body’s internal thermostat, located in the hypothalamus, regulates temperature. When an infection occurs, immune cells release endogenous pyrogens like interleukin-1 (IL-1) and tumor necrosis factor (TNF). These pyrogens travel to the hypothalamus, triggering the production of prostaglandin E2 (PGE2). PGE2 signals the hypothalamus to raise the body’s temperature set point. This prompts the body to generate and conserve heat through shivering and vasoconstriction, leading to the rise in body temperature.
Fever’s Impact on Viruses, Including COVID-19
Fever is a defense mechanism that creates an environment less favorable for viral replication. Elevated temperatures can hinder the ability of viruses, including SARS-CoV-2, to multiply. Studies show that higher temperatures, such as 39°C to 40°C, restrict SARS-CoV-2 propagation in respiratory tissues by inhibiting viral transcription and reducing the release of infectious viral particles. This means fever doesn’t directly “kill” the virus, but it slows its spread by disrupting replication.
The virus’s ability to infect cells and replicate is affected by temperature. SARS-CoV-2, for example, replicates more efficiently at lower temperatures, around 33°C, typical of the upper respiratory tract. Conversely, higher temperatures during a fever decrease overall viral replication. This highlights how fever contributes to the body’s initial response by making it harder for the virus to establish a widespread infection.
The Immune System’s Broader Response to COVID-19
Fever is one part of the body’s comprehensive immune response to viral infections like COVID-19. Beyond temperature elevation, the immune system deploys specialized cells and molecules to combat the virus. This includes antibodies, proteins that neutralize the virus and prevent it from entering host cells. Antibodies play a role in reducing the risk of severe disease.
Another component of the immune response involves T-cells, which directly target and clear infected cells. Cytotoxic T-cells are important for clearing acute viral infections and can provide protection against future infections. Additionally, interferons (IFNs) are signaling proteins that aid antiviral immunity by triggering gene expression that helps cells resist viral infection. An effective interferon response helps contain the virus and prevent its rapid spread.
When to Seek Medical Attention for Fever During COVID-19
While fever is a common symptom of COVID-19 and often resolves within a few days, certain signs indicate a need for medical evaluation. If a fever persists for several days, typically beyond 3 to 4 days, or if it continues to rise, medical advice should be sought. For adults, a temperature of 103°F (39.4°C) or higher can be a sign of a more serious condition.
Immediate medical attention is also warranted if other concerning symptoms develop alongside the fever. These include difficulty breathing, persistent pain or pressure in the chest, new confusion, or an inability to wake or stay awake. Changes in skin color, such as pale, gray, or bluish lips, nail beds, or skin, are also emergency warning signs. Dehydration, indicated by severe thirst or reduced urine output, also requires prompt medical contact.