Recently Spain’s National Centre for Epidemiology completed a systematic randomized survey to estimate the spread of the covid virus through the national population. More than 60 thousand individuals, selected through stratified random sampling, completed questionnaires and were tested for antibodies. Conclusion: 5.2 percent of the Spanish population are seropositive, indicating that at some point they’ve been infected by the virus. That’s a long way from herd immunity.

Spain’s population is 47 million; if 5 percent have been infected, that’s 2.4 million people. By contrast, only 300 thousand Spaniards have tested positive on covid diagnostic tests — one-eighth the actual number of infections.

Divide the number of deaths by the number of infections to calculate the mortality rate. So far 28,400 Spaniards have died from coronavirus; divide that tally by 2.4 million and Spain’s covid mortality rate is 1.2 percent.

**Implications for US**

Based on the small number of rigorous serology surveys previously conducted, I’ve estimated an age-adjusted covid mortality rate of 0.6 percent for the US. “Age-adjusted”: older people are more likely to die of covid than younger people, so a nation with an older average population should experience a relatively higher mortality rate than a younger population. An analysis of the demographics of covid mortality data in the US indicates that each additional year of age increases the likelihood of dying by 10%. The median age in Spain is 45 years; in the US it’s 38 years — a 7-year difference. To calculate Spain’s age-adjusted mortality rate, multiply the estimated US mortality rate of 0.6% by 1.10^{7} = 1.2%: the same as Spain’s serology survey results. The 0.6% mortality estimate for the US is supported by Spain’s data.

How many Americans have been infected by the coronavirus? Divide the number of deaths by the mortality rate to estimate the total number of infections. So far 133 thousand have died; divided by .006 = 22.2 million. The cumulative number of US diagnostic test-positives is 3 million: like Spain, the confirmed diagnoses amount to only about one-eighth of the estimated number of infections.

What percentage of the US population has been infected so far? Divide the 22.2 million infections to date by the national population of 328 million = 6.8 percent. The prevalence in the US is somewhat higher than Spain’s. An early hotspot that has successfully minimized community spread of the virus, Spain has been averaging just 4 deaths per day over the past three weeks.