Hidden immunity in our bones means many of us may not need booster jabs

Anyone keeping an eye on coronavirus antibody levels in Britain may have noticed an alarming trend in recent months. Since peaking in May, the percentage of older people who are testing positive for protective antibodies has been steadily declining. It rose to 95 per cent for the over-80s, and has now fallen to 92.4.

In fact, only the under-50s are not seeing a drop in antibody levels, according to the Office for National Statistics.

This signal of waning immunity has led to calls for booster jabs ahead of a winter wave.

But several studies have come out in the past few months that suggest we may not need to be too worried after all. Antibodies are not the only indicator of immunity.

In May, researchers at Washington University School of Medicine, who studied 77 recovering Covid patients, found that while antibodies declined over time, bone marrow plasma cells, capable of producing antibodies against the virus, remained stable.

It means that people who have recovered from an infection have immunity hidden away in their bones, ready to spring into action. This type of dialled-down protection could last decades, or even a lifetime, the researchers believe.

The team also found patients were carrying memory B-cells – a type of white blood cell – which patrol the blood looking for the virus.

In fact, the researchers noted that the levels of B-cells found in recovered Covid-19 patients were equal to people vaccinated against tetanus or diphtheria, vaccines that both provide long-term immunity to those diseases.

Similarly, researchers from the Rockefeller University, New York, recently found that recovered Covid patients still have immunity a year after an infection, including antibodies that were “exceptionally resistant” to variants.

“The data suggest that immunity in convalescent individuals will be very long lasting and that convalescent individuals who receive available MRNA vaccines will produce antibodies and memory B-cells that should be protective against circulating SARS-COV-2 variants,” the researchers concluded.

The problem with only looking at antibodies is it gives a false picture of the level of protection. The body is efficient, and will not continue to produce high-levels of antibodies when they are no longer needed.

This is a good thing, as it prevents the onset of autoimmunity which can lead to a slew of debilitating conditions. Instead, the blueprints for making antibodies are squirrelled away, while a casual surveillance is mounted by the immune system.

But bone marrow and B-cells are not all the body has stashed away for a future fight. Last year, Imperial College found that people who had recovered from Covid still had memory T-cells even after antibodies had waned. T-cells are a kind of white blood cell that store the details of a past infection and can quickly multiply on reexposure, providing a rapid response to an invader.

T-cells may also have the added benefit of being able to fight variants.

While antibodies only detect proteins on the outside of cells – such as the spike protein that the virus uses to latch on to human cells – T-cells can hone in on proteins inside infected cells. Crucially, many of these do not change when the virus mutates.

Earlier this year, scientists at La Jolla Institute for Immunology in California found that people infected by Covid generate T-cells that target at least 15 to 20 different fragments of coronavirus proteins, and would work even against worrying mutations such as the 501Y.V2 mutation found in the South African (beta) variant.

Infections from the original Sars virus have also produced immunity in recovered patients that has lasted decades. A study by scientists in Singapore, published last week, found that Sars patients who were vaccinated against Covid produced high levels of neutralising antibodies against both viruses.

It suggests that memory regions of the immune system were jogged into action by the vaccine, raising hope that coronavirus infection or vaccination offers long-term protection which can be easily ramped up if the virus re-emerges.

Certainly, we are not seeing any indication that vaccinated people are becoming more at risk for reinfection. Although there have been some breakthrough infections with the delta variant, the vast majority (82 per cent) of people catching Covid currently have not had two doses.

The Joint Committee on Vaccination and Immunisation is expected to advise that booster vaccinations not be rolled out to the wider population, although some more vulnerable people may be offered a third jab. For most of us, it is likely we will be safe enough with just two.

We can generate a type of dialleddown protection that could last decades, or even a lifetime