How well do Covid infections protect against future illness?

Prevalence of the virus has been fairly stable since the beginning of August last year, with infection rates see-sawing between roughly 2-4% at any one time.

READ MORE: Never had Covid? The secret might lie in your genes

That is much higher than it ever was pre-2022, but comparatively low versus the Omicron BA.2 and BA.5 waves seen in the first half of last year when nearly 10% of Scots had Covid at a peak in mid-March.

Omicron continues to dominate, but with an ever-expanding proliferation of sub-lineages: more than half of cases in the UK are now being caused by either the XBB (including the so-called XBB1.5 ‘Kraken’) or C.H.1.1 (nicknamed ‘Orthus’) strains of the variant.

Covid rates in Scotland have fluctuated roughly between 2-4 per cent since August last year, with signs of a slight rise again in the most recent week (Image: Office for National Statistics)

Both appear to have mutations which enable them to bind to cells even more easily and dodge antibodies, giving them a transmission advantage.

There is no evidence that they cause more severe disease, however, and vaccines continue to provide strong protection against hospitalisation and death - as, it appears, do prior infections.

The current Covid backdrop comes as the most comprehensive study to date has investigated to what extent prior exposure to the virus defends against subsequent Covid illness.

It has to be stressed that scientists are not proposing natural immunity as an alternative, or equal, to vaccination - the latter is by far the safest way to build protection - but the study, published in the Lancet, provides useful insight, particularly given that most under-50s are not currently eligible for any additional booster shots and the likelihood of repeat exposure is high.

The meta-analysis pulls together data from 65 studies in 19 countries - including Scotland - to evaluate for the first time how well, and for how long, a Covid infection protects against reinfection and severe disease (hospitalisation and death) from subsequent bouts across different variants.

Studies examining natural immunity in combination with vaccination (“hybrid immunity”) were excluded, meaning that the risk related specifically to unvaccinated individuals.

READ MORE: Why Covid reinfections could be a bigger problem than expected

Researchers found that the level and duration of protection against reinfection, symptomatic disease and severe illness is "at least on a par with that provided by two doses of the mRNA vaccines" (Moderna, Pfizer-BioNtech) in relation to the “ancestral” form of the Covid virus (the original strain which emerged from China in late 2019) and the subsequently dominant Alpha, Delta, and Omicron BA.1 variants.

The analysis does not cover the more recent forms of the virus from Omicron BA.2 up to the current XBB or C.H.1.1 strains.

Collating the findings of 21 studies reporting on pre-Omicron variants, the scientists estimated that protection against reinfection had fallen slowly from around 85% at one month post-infection to 79% after ten months.

Africa experienced substantial waves of Covid and was at the epicentre of the Omicron wave, but deaths have been comparatively low (Image: Getty)

For people infected with one of the pre-Omicron variants (typically Delta) who were then exposed to Omicron BA.1 when that wave took off towards the end of 2021, protection against reinfection was lower: 74% at one month and declining more rapidly to 36% ten months on.

This ability of Omicron to cause reinfections within a short period is at least part of the reason why the virus has remained at such stubbornly high rates over the past year (along, of course, with the absence of nearly any mitigations to slow its spread).

In relation to severe disease, however, protection was much more robust.

An analysis of five studies found that a prior infection considerably reduced the probability of hospitalisation and death for 10 months regardless of the variant: 90% for the ancestral, Alpha, and Delta strains, and 88% for the Omicron BA.1 variant.

In other words, for an unvaccinated person previously infected with Covid-19, their risk of being hospitalised or dying as a result of Covid was 88% lower 10 months on compared to an unvaccinated person who had never had a Covid infection.

READ MORE: Obesity, deprivation, women - and the surprising cost of 'being too fat'

None of this means that we could have foregone mass vaccination in favour of herd immunity.

As lead author, Dr Stephen Lim, of the University of Washington School of Medicine, notes: “Acquiring natural immunity must be weighed against the risks of severe disease and death associated with the initial infection.”

Reports of huge death tolls in China during the recent wave are a warning of what is at stake when you try to open up a society to Covid without high levels of vaccine protection, particularly among the elderly.

It is at least a reminder, however, of the way the human immune system has evolved over millions of years to manage virus threats.

Separate research by Edinburgh University into Covid in Zimbabwe - also published this week, in the journal Lancet Microbe - suggests that a higher prevalence of common cold viruses in Africa may have helped to curb the continent’s death toll from the disease.

To date, Africa has reported 180 Covid deaths per million compared to 2,720 per million in Europe, despite high incidence and lower vaccination rates.

Confirmed Covid deaths rates have been much higher in Europe than Africa despite lower vaccine coverage (Image: Our World in Data)

While its warmer climate and more youthful population will have been important, scientists believe that the population may have also benefitted from higher levels of “cross-reactive immunity” - antibodies accumulated in their blood in the years before the pandemic as a result of regular contact with other coronaviruses which circulate at higher levels in Africa compared to other parts of the world.

It is thought that some of these antibodies, produced in response to pre-existing coronaviruses, were also able to defend against the distinctive “spike protein” found in Covid.

The same phenomenon (prior exposure to other coronaviruses) is believed to explain why some people have been less likely than others to catch Covid even in highly vaccinated counties such as Scotland.