The average time from the start of symptoms to developing critical illness is seven to eight days. Only a minority of people (about 20%) who develop symptoms of COVID-19 will become seriously ill, requiring hospitalisation. See also How does the coronavirus cause illness?. If we know in advance who they are, they can be given extra protection, monitored closely and, if necessary, cared for in hospital. It is also useful to know who is at low risk of serious illness because they are less likely to need medical attention.
This article summarises current evidence about the conditions, including ageing, that are associated with increased vulnerability to severe COVID-19 infection. Specific advice to vulnerable people, including self-isolation, can be found elsewhere, and is regularly updated as new information arises.
Older people have a higher risk of becoming seriously ill with COVID-19 and needing intensive care. This could be because, as people age, their immune systems become less able to fight off new infections. Older people’s lungs, heart and other vital organs are also more susceptible to damage caused by the virus. But it is also the case that older people are more likely to have one or more of the chronic underlying conditions associated with an increased likelihood of more severe symptoms. In contrast to the elderly, children and younger people generally exhibit mild symptoms, and severe complications appear to be uncommon.
People who have high blood pressure, diabetes, chronic lung disease and heart disease are more likely to become seriously ill with COVID-19. It is possible that these illnesses make COVID-19 worse because damaged organs perform less well when stressed by the impact of the virus. However, all these illnesses tend to occur in older people, so age itself could be playing a part in making COVID-19 worse in people with underlying health conditions.
Immunosuppression is when the function of the immune system is reduced. This can be due to genetic reasons, for those who have had organ transplants, or because of other illnesses, for instance infection with HIV/AIDS, or severe kidney disease. It can also be caused by medicines, for instance steroids taken for chronic inflammatory diseases such as rheumatoid arthritis and multiple sclerosis, as well as chemotherapy. Any of these conditions could increase the risk of severe disease, based on evidence from other viral infections.
Men could be at greater risk than women of severe disease, but the difference seems to be small and has not been seen in all studies. The reasons why men might be at greater risk are unclear. One possibility is that men tend to smoke more than women, but there is presently no strong evidence of an association between smoking and the frequency or severity of COVID-19 in men.
Patients with chronic conditions, who are at relatively high risk of developing severe COVID-19 symptoms, are frequently taking a number of medications. This raises the question as to whether it is the condition or the medication that is causing more severe illness.
There is evidence, for example, that patients with chronic heart disease, diabetes and high blood pressure are more susceptible to severe COVID-19 symptoms. These conditions can be controlled with medicines such as angiotensin-converting enzyme (ACE) inhibitors or angiotensin receptor blockers. So far, there is no evidence that these medications are harmful for patients with COVID-19 and a number of specialist medical organisations have issued guidance to this effect.
It has also been suggested that the non-steroidal anti-inflammatory drug, ibuprofen, could make COVID-19 symptoms worse. There is presently no evidence of any adverse effects of this medication in patients with COVID-19. However, while uncertainty prevails, the prudent course of action is to use treatments for headache, fever and inflammation that are known to be safe and effective, such as paracetamol, unless medical advice is to the contrary.
Myths arise quickly in the context of treatments for a new disease. The articles on this website will continue to update the evidence on what treatments are effective, what treatments are ineffective and what is uncertain.
It has long been known that pregnancy can change how the body copes with infections such as influenza. An expectant mother and the developing fetus might also be at increased risk of COVID-19. So far, however, there is no evidence that pregnant women are more likely to contract the disease, or that the disease is any more severe in pregnancy. It is not known if a woman with COVID-19 can pass the virus to her child during pregnancy, and the virus has not been found in breast milk.