The results were also published in the medical journal, the Lancet, which said that the findings showed that attaining herd immunity was impossible to reach without an unacceptable level of death and suffering as health systems collapse.
In the conclusions to its peer review of the first round of the Spanish study, carried out from late April to early May, the Lancet said that the low Covid-19 prevalence even in a badly-affected country such as Spain meant that herd immunity "cannot be achieved without accepting the collateral damage of many deaths in the susceptible population and overburdening of health systems".
It said this meant that social distancing and track and trace efforts would continue to be "imperative" in controlling the pandemic.
The first round of the Spanish study recorded 5 per cent of the population showing antibodies for Covid-19.
It also showed a mortality rate of between 1.1 and 1.5 per cent, depending on whether officially confirmed fatalities due to Covid, or all suspected and known cases, were counted.
Of those who tested positive in the survey, 26 per cent had not noticed any symptoms.
The three-phase study, involving many repeat participants, also showed how antibodies quickly stopped showing up in many cases, with 14 per cent of those who tested positive for antibodies in the first wave coming out negative two months later.
"Not being able to detect antibodies does not mean that these people are not immunologically protected," said Pollán, explaining that people can also have cellular immunity, which has yet to be studied closely.
Some experts have suggested that T-cells, a type of white blood cell that helps the immune system fight off viruses, may have as important a role to play in taking on the virus, but more research is needed.
Spain's official death toll from Covid-19 stands at 28,385, but the real number is thought to be considerably higher, and the country's excess death figure from the beginning of March to mid-May is close to 43,000.
The prevalence study was carried out to determine how many people in Spain have developed antibodies after exposure to the virus.
The eight-week study was conducted by researchers at the Carlos III public health institute, which took blood samples from close to 70,000 participants.
