Coronavirus: First antibody study indicates that just 4.3% of Scots have had Covid-19 infection so far

Detection levels varied each week between 1.9% and 6.8%, said PHS.

The presence of antibodies indicates that a person has been exposed to and recovered from an infection, and can give a clearer gauge on the spread of a virus because they will also occur in cases where the individual had mild or no symptoms.

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Antibodies should also confer immunity against re-infection, although research increasingly suggests that in the case of Covid-19 this can disappear in around three months - maybe even less.

A total of 4,751 blood samples were collected during the pilot study from people attending community healthcare services in six health board areas: Greater Glasgow and Clyde, Grampian, Highland, Lanarkshire, Lothian, and Tayside.

Samples were selected to achieve a representative sample based on the age and sex structure of the general population in each area, although PHS said there was still “uncertainty whether these individuals are representative of the general population”.

Rates of antibodies were similar among males and females across the study period.

If applicable to the Scottish population as a whole, it suggests that around 234,500 people may have been infected by the virus so far.

That would put the overall mortality rate from the virus at between 1.1 and 1.7%, depending on whether deaths are counted using only confirmed positive cases or on the basis of death certificates where Covid is mentioned.

Dr Nicola Steedman, Scotland's interim deputy chief medical officer, discussed the significance of the study at the daily virus briefing on Thursday.

She said many people who have coronavirus will not show symptoms and will therefore not be tested, describing this group as the "bottom of the iceberg".

However everyone who has contracted Covid-19 will produce antibodies, she said, giving a more complete indication of how the virus has spread through the population.

Dr Steedman said: "Only a fairly small proportion of the population have so far likely been exposed to coronavirus in Scotland.

"And it's this low number of people exposed that explains and reinforces our ongoing messages to you."

She reiterated the Government's public health messaging, saying: "Firstly, we need to be careful when we're easing out of lockdown and secondly this is why we still want you to follow the current guidance on physical distancing and all the other measures that we recommend in order to protect you, your loved ones and, in fact, to protect all of us."

The results for Scotland are roughly in line with antibody rates observed in other countries.

The largest European study to date, of 61,000 people in Spain, found that just 4.6% of participants tested positive for Covid-19 antibodies despite the country’s high death toll.

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The authors, writing in the Lancet last week, warned that the findings should be a warning against trying to achieve herd immunity through natural infection.

They said: “The relatively low seroprevalence [antibodies present in blood] observed in the context of an intense epidemic in Spain might serve as a reference to other countries.

“At present, herd immunity is difficult to achieve without accepting the collateral damage of many deaths in the susceptible population and overburdening of health systems.”

It is also unclear from research whether Covid-19 antibodies offer any long-term protection against falling ill with the virus again.

The Spanish study showed that around 14% of participants who tested positive for antibodies initially had lost them after eight weeks.

Meanwhile, a UK study published this week of more than 90 patients and healthcare workers who had recovered from Covid found that antibody levels peaked around three weeks after the onset of symptoms, but in some cases were no longer detectable after three months.

It is unclear whether people develop alternative lines of defence - such as cellular immunity - but if antibodies are the main protection it suggests that people could become reinfected in seasonal waves, as happens with the common cold.

It could also mean that a Covid vaccine will offer only temporary immunity.

Dr Katie Doores, lead author of the study at King’s College London, said: "Infection tends to give you the best-case scenario for an antibody response, so if your infection is giving you antibody levels that wane in two to three months, the vaccine will potentially do the same thing.

“People may need boosting and one shot might not be sufficient.”

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The antibody findings for Scotland came as Nicola Sturgeon spoke of her nervousness as indoor hospitality, hair salons, and communal worship resumed in Scotland for the first time since March, stressing that these were "by some margin the highest risk changes we have made since we started to come out of lockdown".

"Many of them involve indoor activity, and we know that the risk of the virus spreading indoors - in a pub for example - is significantly higher than outdoors," she said.

"That is why we have deliberately waited until infection levels are very low before allowing these services to restart.

"That gives us the best possible chance of managing the risks that a reopening of indoor services creates. But it doesn’t remove those risks."

She said it was more important than ever to be "hyper-vigilant" to any early warnings that cases were on the rise.

In addition to Test and Protect, she said NHS 24 call data would be analysed for signs of possible community transmission based on people phoning the helpline with Covid-like symptoms.

However, the First Minister also hailed a "really significant milestone" after Scotland recorded three days with zero Covid-related hospital admissions last week.

She also reported that there had been no deaths for seven consecutive days among patients who had tested positive for the virus, while National Records of Scotland registered 13 fatalities where Covid was mentioned on the death certificate in the week to Sunday - the lowest number since mid-March.

The total number of people dying in Scotland, from all causes, has also continued to fall.

Over the past three weeks, 116 fewer deaths than average have been registered - including 56 last week - although it is possible that this is because some sick and elderly people died prematurely earlier in the year due to the pandemic.