Covid-19 vaccines work. Data from Israel and Scotland show they are protecting people and may also reduce the spread of the SARS-CoV-2 virus. If everything resists, people will be protected from serious disease, the amount of the virus will gradually decrease, and we can really plan for a way out of the pandemic.
It’s also increasingly clear that once you’ve been infected there’s a good chance you’ll be protected against other infections or, at the very least, have less serious illness. This makes sense, because that’s why your immune system evolved in the first place.
For millions of years, the immune system has been shaped by the laws of natural selection. Once you have been infected or vaccinated, B and T memory cells persist. When you are re-infected, they wake up and clear the infection so much that you won’t even feel sick. You can see how it made sense to evolve. Feeling sick means you are less likely to father a child, so there would have been a survival benefit to immunity.
However, an important question in immunology, when it comes to infectious diseases and vaccines, is: how long could protection last? There are several variables here, from the type of pathogen infecting you to the severity of the initial illness, your general health, and your age. All of this makes it difficult to predict what could happen with Covid-19.
Measles or flu?
It may be useful to compare what we currently know about Covid-19 with two diseases that we know a lot about and for which we have effective vaccines: measles and influenza. In the future, what will Covid-19 look like?
Measles is a much more stable virus than SARS-CoV-2 – it doesn’t mutate much. It also usually causes a strong immune response, so immunity tends to last a long time, whether from infection or from vaccines. One study has shown that antibodies to measles last a lifetime. Because this is such a stable virus, when it re-infects itself, the vaccine you may have received years ago, or the response to natural infection, will protect you.
The flu, however, is different. The flu virus can change easily. This means that we have to keep vaccinating against it, because vaccines from a previous variant may not protect against a new variant. The stability of a virus is therefore a key determinant of the persistence of protection.
Covid-19 likely sits somewhere between measles and the flu. It is not as stable as measles and it is not as progressive as the flu. Immunity can be expected to last against Covid-19, but probably not as long as measles. And we will probably need to vaccinate against the variants that emerge, as we do with the flu.
One thing we have going for us is the repetitive nature of the SARS-CoV-2 surface. The spike protein covers the surface of the coronavirus fairly evenly. Smallpox antibodies, which also have a very repetitive surface, last a lifetime. In this situation, macrophages (a type of white blood cell that engulf and consume pathogens) might be more able to engulf the antibody coated virus.
If the spike protein mutates and the antibodies can’t bind as well, it’s worth giving the new spike protein a boost – which is expected.
And if the antibodies don’t work as well against the variants, the T cells might. This could mean that we won’t need any boosters at all and may have long-term protection against multiple variants. And even if the immune response is weaker against the variants, we will probably still be protected against serious illnesses.
An important aspect of natural infection is the strength of the initial immune response. The common cold often causes only a mild response in the upper respiratory tract.
This is because a virus that is confined to your nose is not much of a threat. This means that the immune response doesn’t really kick in at all. It is not enough for B and T memory cells to emerge.
If the flu causes a big fight, which the immune troops never forget, the common cold is a simple skirmish that is quickly forgotten. A moderate dose of Covid-19 could be similar. If you’ve had a more severe illness, this might help and make you more resistant to re-infection. But if you only had mild illness or were symptom-free, you are at risk of re-infection.
Where vaccines make sense is their power. They usually give a much stronger immune response than a natural infection. This is because natural immune responses lack the cunning of the pathogen, many of which have elaborate ways to turn off the immune response. It is still a question of evolution.
Viruses that carry proteins that can limit immunity are more likely to survive. This can be especially important with SARS-CoV-2, which has several ways of limiting immunity. Since vaccines include either part of the virus – like the spike protein – or an entire inactivated virus, they do not limit immunity and a robust immune response occurs. The Moderna vaccine, for example, has been shown to elicit a long-lasting antibody response, where natural infection is more variable.
We are now convinced that infection with SARS-CoV-2 is highly likely to provide some protection against reinfection. But, since we are seeing variations, it is wise to prepare for booster shots with new vaccines for vulnerable people. We will have a better idea of their need in the months to come.
If they are, Covid-19 will be more like the flu, which needs boosters. But if they are not, then it will be more like measles, where the only threat is to those who refuse vaccination.
Luke O’Neill is Professor of Biochemistry at Trinity College Dublin.
This article first appeared on The Conversation.