Fever, or pyrexia, is a widely recognized response to viral infection, yet many people who contract COVID-19 never develop an elevated temperature. Fever is typically defined as a body temperature above 100.4°F (38°C) orally. The absence of fever does not mean the virus is not present or cannot cause illness, highlighting the complexity of the body’s immune reaction to SARS-CoV-2. Understanding why this common defensive reaction is missing involves examining the fundamental biology of the fever process.
The Immune System’s Fever Response
The body raises its temperature as a deliberate defense mechanism against infection, regulated by the hypothalamus, the brain’s temperature control center. When the immune system detects a pathogen, specialized immune cells release inflammatory signaling molecules called endogenous pyrogens (e.g., interleukins and tumor necrosis factor). These pyrogens travel to the hypothalamus, triggering the production of prostaglandin E2 (PGE2), which resets the body’s internal thermostat to a higher point.
This upward adjustment prompts the body to generate and conserve heat through shivering and peripheral vasoconstriction. The elevated temperature enhances immune cell function and creates an unfavorable environment to slow viral reproduction. The fever eventually drops when the stimulus is removed or when the body produces anti-pyretic cytokines to lower the set point, leading to sweating and heat loss.
Factors Contributing to Absent Fever
The absence of fever often relates to the host’s immune history and the specific characteristics of the infecting virus. Prior immunity, whether from vaccination or a previous infection, leads to a quicker and more effective immune response. This immune memory allows the body to neutralize the virus, resulting in a milder case less likely to trigger the massive inflammatory cascade that causes fever.
The particular variant of SARS-CoV-2 also influences the likelihood of fever. Newer variants, especially the Omicron lineage, replicate more efficiently in the upper respiratory tract rather than the lower lungs. An infection localized to the upper airways is less likely to generate the high systemic inflammatory signals needed to reset the hypothalamic thermostat.
Common COVID-19 Symptoms Without Fever
A lack of fever does not indicate the absence of infection, as many people experience a constellation of other symptoms with COVID-19. Fatigue is a very common complaint, often described as a profound tiredness disproportionate to the mildness of other symptoms. Body aches, or myalgia, often present as generalized muscle soreness and discomfort.
Other frequent manifestations resemble a common cold, including a sore throat, congestion, and a runny nose. While the loss of taste or smell was characteristic of earlier strains, it is less common with newer variants but remains a possibility. These symptoms confirm the body is actively fighting the virus, even if fever is not engaged.
When to Seek Medical Guidance
The absence of a fever can be particularly misleading for certain groups who naturally have a blunted immune response. Older adults and immunocompromised individuals may have an impaired ability to generate the inflammatory response required for a fever, meaning a serious infection can be present without a temperature spike. Additionally, the use of common over-the-counter fever-reducing medications, such as acetaminophen or ibuprofen, can mask a low-grade fever.
Warning Signs of Severe Illness
Regardless of temperature, certain symptoms warrant immediate medical attention because they suggest a progression to severe disease. Difficulty breathing or persistent shortness of breath is a sign of respiratory distress requiring urgent evaluation. Other warning signs include:
- Persistent pain or pressure in the chest.
- New confusion.
- Inability to wake up or stay awake.
Monitoring for these symptoms, which indicate low oxygen levels or severe systemic illness, is much more important than relying solely on the presence or absence of a fever.