Short-term changes in a person’s immune defenses are normal when they have been exposed to an infection
Over the last month or two, many countries in the northern hemisphere, including the US and the UK, have seen a large wave of respiratory viral infections. These include RSV (respiratory syncytial virus), influenza, and COVID at all ages, as well as bacterial infections such as strep A in children.
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Sometimes these infections can be very serious. The UK has seen a huge increase in hospital admissions over the winter, putting the health service under further pressure.
This led some to wonder if COVID damages our immune systems, leaving those who have been infected more vulnerable to other infectious diseases like the flu.
Another idea put forward to explain the rise in respiratory viruses is that children “missed out” on common childhood infections during the height of the pandemic, and that this has left them more vulnerable to these infections now due to “immunity debt.” “. But how credible are these explanations?
COVID and our immune system
The human immune system has evolved to deal with a large number of different infections. You have a variety of weapons you can deploy that work together to not only eradicate infectious agents, but also recall them for a faster, more personalized response in any subsequent encounter.
Likewise, many infectious agents have developed tricks to try to evade our immune systems. For example, a parasite called Schistosoma mansoni disguises itself to avoid detection by the immune system.
SARS-CoV-2, the virus that causes COVID-19, also has tricks up its sleeve. Like many other viruses, it has been shown to evade host immunity, particularly newer variants. Recent studies have shown that it can interfere with the ability of immune cells to detect it inside cells. This is concerning, but it is not clear that such changes affect immunity to other infections.
Short-term changes in a person’s immune defenses are normal when they have been exposed to an infection. Several studies have now shown that, in response to SARS-CoV-2, specialized white blood cells called lymphocytes increase. These lymphocytes also show changes in their typical characteristics of cell activation, such as changes in surface proteins.
Such changes can sound dramatic to the non-expert if taken out of context (called “verification bias”). But they are normal and simply indicate that the immune system is working as it should. Research has confirmed that for most people, the immune system returns to balance after recovery.
SARS-CoV-2, like many viruses, does not affect everyone equally. We have known for some time that certain groups, including older people and those with underlying health complications such as diabetes or obesity, may be more susceptible to serious illness when they contract COVID.
This vulnerability is associated with an irregular immune response to SARS-CoV-2 that results in inflammation. Here we see, for example, a reduced number of lymphocytes and changes in immune cells known as phagocytes.
Still, for most of these vulnerable people, the immune system returns to normal within the next two to four months. However, a small subset of patients, particularly those who had severe COVID or underlying medical problems, retain some changes beyond six months after infection.
The significance of these findings is unclear, and longer-term studies that consider the impact of underlying health conditions on immune function will be needed. But for most people, there is no evidence to suggest immune damage after a COVID infection.
What about the long COVID?
Emerging evidence suggests that the most marked and persistent differences in immune cells after a COVID infection occur in people who have had COVID for a long time.
So far, no data points to an immune deficiency in patients with long-term COVID. But an overactive immune response can actually cause harm, and the immune cell changes seen in long-term COVID patients appear to be consistent with a vigorous immune response. This may explain the range of post-infection consequences and symptoms faced by people with long-term COVID.
The “immunity debt” hypothesis suggests that the immune system is like a muscle that requires almost constant exposure to infectious agents to maintain its function. So, the argument goes, lack of exposure due to lockdowns damaged immune development, especially in children, by causing our immune systems to “forget” previous knowledge. This supposedly left them more vulnerable to infection when the social mix returned to normal.
Although this idea has gained ground, there is no immunological evidence to support it. It is not true to say that we need a consistent history of infection for our immune systems to function. Our immune systems are immensely robust and powerful. For example, the immune memory of the 1918 influenza pandemic was still evident after 90 years.
It is also not strictly true to say that children were not exposed to viruses during the early pandemic. Closures did not start until after the usual waves of winter respiratory infections in 2019/2020, and UK schools reopened in autumn 2020 with varying preventative measures, leaving children still exposed to infections, including COVID-19 .
The viruses that cause the cold did not completely disappear by any means. For example, there was a significant RSV outbreak in the UK in 2021.
However, lockdowns and other protective measures likely reduced exposure to viruses, and for some children this changed when and at what age they were first exposed to viruses such as RSV. This, coupled with a high COVID history and relatively poor flu and COVID vaccine response, could be making this season particularly bad. However, a change in the timing of exposure that leads to a surge of infections does not necessarily mean that individual immunity has been damaged.
Our knowledge of the immune response to COVID is expanding rapidly. The most consistent findings show how well vaccines protect us from the worst effects of SARS-CoV-2 and that, after vaccination, our immune systems work exactly as they should.
However, the findings of altered immune signatures in some recovered patients and those with prolonged COVID require further investigation.
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