The immune response in a person with non-severe COVID-19

Scientists in Melbourne, Australia, were able to analyze the immune response of a woman with non-severe COVID-19. They found that the immunological response was both fast and dynamic. The findings from their study can help support a better understanding of SARS-CoV-2 infection and the type of responses needed from a future candidate vaccine.

Case details

A 47-year-old woman from Wuhan, China, sought care at a Melbourne hospital because for four days she was experiencing the following: lack of energy, sharp chest pains when breathing, mild shortness of breath and fever. Eleven days prior she had visited Wuhan. She was a non-smoker and otherwise healthy and not taking medicines.

Doctors found that she had a mild fever, a rapid pulse and somewhat elevated blood pressure. These can be general reactions to an infection. The amount of oxygen in her blood was normal.

On the day of admission to the hospital, swabs from her nose and throat tested positive for SARS-CoV-2. The doctors called this day four, as she had a four-day history of symptoms. The virus continued to be detected on days five and six in samples from her nose, throat and stool. However, on day seven, no virus was detected, nor was it on any subsequent day.

Analysis of the woman’s blood samples found normal levels of lymphocytes and other cells of the immune system. However, her overall level of inflammation was high.

No other respiratory germs were detected and the sounds that she made when breathing were normal. A chest X-ray on day five showed mild inflammation that resolved on day 10.

As her symptoms of COVID-19 were considered “mild-to-moderate,” doctors did not prescribe antibiotics, steroids or antiviral drugs.

On day 11, the woman was sent home and told to remain in isolation. Doctors found that her symptoms entirely resolved on day 13. On day 20, the end of their observation of her, she continued to be well.


Further laboratory analysis of the woman’s stored blood samples revealed that antibody producing cells appeared on day seven, the same day that virus was no longer detected. Subsequent testing revealed that the level of these cells reached their peak on day eight and then declined.

Levels of antibodies (called IgG) that attacked SARS-CoV-2 were detectable on day seven and increased through day 20. Levels of another type of antibody (called IgM) that also attacked the virus became detectable on day nine and rose through day 20.

Antiviral CD8+ cells

During acute viral infections, CD8+ T-cells are activated and deployed by the immune system to attack viruses and virus-infected cells. On day seven, the level of activated CD8+ cells rose and kept rising but had fallen by day 20. The level of activated CD4+ T-cells also rose during the same period but not as much as CD8+ cells did.

Scientists also assessed the functioning of CD8+ cells, specifically, their ability to release enzymes that damage the genetic material of virus-infected cells and cause them to die. Activated CD8+ cells were highly functional.

No increased levels of other cells, natural killer cells or monocytes/macrophages were found.

Signals of inflammation

Excessive levels of inflammation may play a role in some viral infections when organ injury occurs. In the present study, scientists found that levels of chemical signals that incite inflammation were “minimal” in the blood samples of the woman. This was the case even on days seven through nine when she had symptoms of COVID-19.

Bear in mind

The present study underscores the dynamic nature of the immune response; it can wax and wane. The study also shows the need for similar studies in larger numbers of people throughout their course of COVID-19. Such studies may help do the following:

  • identify components of a useful immunological response against SARS-CoV-2
  • uncover the timing of such a response, which has implications for treatment (once treatments are approved)
  • contribute to the development of candidate vaccines against SARS-CoV-2

—Sean R. Hosein


Thevarajan I, Nguyen THO, Koutsakos M, et al. Breadth of concomitant immune responses prior to patient recovery: a case report of non-severe COVID-19. Nature Medicine. 2020;26(4):453–455.