What is the likelihood of being contagious with a common cold (upper respiratory tract infection) 4 days after symptom onset, despite some improvement in symptoms?

From the Guidelines

You are likely still contagious, but your contagiousness has probably decreased significantly since you've had symptoms for 5 days and are feeling somewhat better. At this point, you are likely less contagious than you were earlier in your cold, but you may still be able to spread the virus. Most common colds are most contagious during the first 2-3 days of symptoms, and contagiousness typically decreases significantly after about 5 days. Since you've had symptoms for 5 days and are feeling somewhat better, your contagiousness has probably decreased, but the persistent congestion suggests you're still shedding some virus. To be cautious, continue practicing good hygiene like frequent handwashing, covering coughs and sneezes, and considering wearing a mask around vulnerable individuals for another 1-2 days. Cold viruses spread through respiratory droplets and contact with contaminated surfaces, so these precautions help reduce transmission. According to a study on the prevention and control of influenza 1, adults can be infectious from the day before symptoms begin through approximately 5 days after illness onset, which supports the idea that your contagiousness is decreasing. Most people are no longer contagious after 7-10 days from symptom onset, so you're approaching that window. If your symptoms worsen again or you develop fever, your contagious period might be extended, and you should reassess. Some key points to consider include:

• The typical incubation period for influenza is 1--4 days, with an average of 2 days 1
• Adults can be infectious from the day before symptoms begin through approximately 5 days after illness onset 1
• Children can be infectious for >10 days after the onset of symptoms, and young children also can shed virus before their illness onset 1
• Severely immunocompromised persons can shed virus for weeks or months 1

From the Research

Contagiousness of a Cold

• The provided studies do not directly address the contagiousness of a cold, but study 2 discusses the duration of infectiousness of individuals infected with SARS-CoV-2, which may provide some insight into the general duration of infectiousness for respiratory illnesses.
• According to study 2, COVID-19 patients with mild-to-moderate illness are highly unlikely to be infectious beyond 10 days of symptoms.
• However, it is essential to note that the common cold is caused by different viruses, such as rhinoviruses, coronaviruses, adenoviruses, and others, and the duration of infectiousness may vary depending on the specific virus and individual factors.
• Since you started developing symptoms of a cold on Monday, April 21, and it is now Friday, April 25, you are likely on the fifth day of your illness.
• Although the studies do not provide direct evidence on the contagiousness of a cold, it is generally recommended to assume that you are contagious for the first 7-10 days of a cold, with the highest contagiousness typically occurring in the first 2-3 days.

Symptoms and Recovery

• You mentioned that you still have congestion, which is a common symptom of a cold.
• Studies 3, 4, 5, and 6 discuss the treatment of pain and fever with ibuprofen and paracetamol, but they do not provide information on the contagiousness of a cold or the duration of symptoms.
• It is essential to continue practicing good hygiene, such as frequent handwashing, covering your mouth and nose when coughing or sneezing, and avoiding close contact with others, to reduce the risk of transmission.