What do we know about coronavirus immunity?

More than 10 million people in the UK have received their first dose of a coronavirus vaccine.

Much has been said about how effective the jabs are at preventing people from getting Covid-19 and when enough people will be protected for things to start returning to normal.

The British Society for Immunology (BSI) says understanding immunity to Covid-19, induced both by natural infection and through vaccination, is key to exiting the pandemic.

Here is what we know about Covid-19 immunity:

– Does everyone have the same immune response to the virus?

No. According to the BSI Covid Taskforce, some people appear to have a very effective immune response and cannot be infected again.

This means they will not get sick again from the virus and will not pass it to anyone else – so called sterilising immunity.

Others make antibodies and are protected from disease, but not from future infection. These people could still pass the virus on to other people.

How many antibodies people make, and how good they are at preventing infection, also varies from person to person.

They also vary in the number of T-cells – a type of white blood cell which seeks and destroys infected cells – and the quality of the response.

It is not yet known how important these variations are in terms of protective immunity, but research is ongoing.

– How do we know if someone has encountered the virus and had an immune response?

As things stand, the easiest indication is whether they have antibodies in their blood which recognise the virus.

Memory T cells and memory B cells that can recognise the unique components of the virus also indicate prior infection but are harder to measure.

The presence of antibodies does not always predict the presence of specific T cells or memory B cells.

– What does effective immunity to coronavirus look like?

The initial immune response appears to correlate with severity of disease – the more severe the illness someone experiences, the stronger their immune response.

However, in some cases the immune response itself is pathogenic, for example producing antibodies that damage tissue, or inducing excessive inflammation.

– What proportion of infected individuals mount an effective immune response?

Studies suggest the majority of people who have a confirmed infection have some form of immune response but whether this is effective at preventing reinfection is not easy to determine.

Research indicated a large majority of recovered patients have detectable antibodies for several weeks or months after their infection.

– Can immunity be lost?

Yes. Antibody levels can decline over time.

But even if antibodies are lost, immunity can be reactivated. Antibody-secreting cells can be renewed from the memory B cell population, with help from memory T cells, if the person encounters the virus again (either naturally or by vaccination, or booster vaccination).

– How long does this immunity last?

Currently, the best marker of protection from future disease is the level of neutralising antibody in the blood.

This is the component of the antibody repertoire that can stop viral entry into cells.

While a decline in neutralising antibody is seen, it can persist in individuals for at least eight months, and possibly longer, after infection, research has indicated.

– So could someone with an immune response be given an immunity passport and allowed to carry with life as normal?

Because each person has a different immune response, and immunity may wane over time, an immunity passport could only be issued if there were accurate ways of measuring immunity to infection and knowing how long it will last.

Experts say there is not enough data to accurately predict an individual’s level and duration of protection from Sars-CoV-2 for an immunity passport.

– Can people get reinfected with coronavirus?

Yes, but it is not very common.

Reinfection depends upon the level of immunity a person gains from the first infection and the level of likely exposure due to social and working environments.

– What kind of protection do Covid-19 vaccines provide?

So far we know the vaccines from Pfizer/BioNTech, Moderna, Oxford/AstraZeneca, Novovax and Johnson & Johnson are effective at stopping people from getting sick.

Not much is known about whether the jabs stop transmission of the virus, but data from the University of Oxford suggests the AstraZeneca vaccine may reduce transmission by 67%.

– Will vaccines work on everyone?

Some people might not be able to generate an immune response after vaccination.

It is thought older people and immuno-compromised people might not have as good an immune response to coronavirus infection, and they may similarly be less likely to see as strong an immune response to a vaccine compared with someone young and healthy.

However data from Pfizer/BioNTech, Moderna and Oxford/AstraZeneca suggests vaccines generate similar immune responses in older and younger people.

There is limited efficacy data for older individuals at present, but this will continue to be evaluated.

None of the trials enrolled immuno-compromised individuals, so there is no data evaluating the immune responses or efficacy in these groups.

But vaccination is still recommended as immuno-compromised people may be at higher risk of severe Covid-19.

– How long will immunity from vaccination last?

This is not yet known, but it is thought it will be at least a year. Researchers agree it is likely people will need to be revaccinated every year.

– How do we achieve herd immunity?

Herd immunity is when transmission of the virus within a population is markedly reduced due to the high proportion of people who are already immune either through vaccination or prior infection.

If enough people in the population are immune, the virus remains at low or undetectable levels, thus protecting anyone who is not yet vaccinated.

In essence, the number of people required to have immunity is deduced from a mathematical formula dependent on the R value for the virus.

In the case of coronavirus and vaccine efficacies approximately equal to 95%, it is estimated that 63–75% of people need to be immune to provide herd immunity.

A high uptake of vaccines, or widespread natural immunity following infection, is needed to achieve this.

Some commonly used words when talking about immunity, and what they mean:

Antibodies – large Y-shaped proteins produced by B cells. They act to neutralise invading pathogens such as the Sars-CoV-2 virus. They can also signal to other cells to help them recognise pathogens.

Antigen – a substance that triggers the body to produce antibodies against it.

B cell – a type of white blood cell that produces antibodies as part of the adaptive immune system.

T cell – also known as T lymphocytes, T cells are a type of white blood cell that determines the specificity of immune response to antigens in the body.

Adaptive immunity – a subsystem of the immune system which comprises specialised cells and processes, and removes pathogens by preventing their proliferation in the body.

Protective immunity – immunity conferred by an immune response which gives protection against an infectious disease. It will not prevent person-to-person transmission.

Sterilising immunity – immunity in which the immune system can prevent the replication of a pathogen within the body. It will prevent person-to-person transmission.

Pathogen – any micro-organism which can cause disease in a host organism.