The medical journal Lancet has published a new study looking at the seroprevalence of the coronavirus in Spain. The study used a large, nationwide sample of more than 61,000 people and gave them two different types of blood tests to determine if they had antibodies to the virus. Using the two tests in combination they came up with a “seroprevalence range,” i.e. an estimate of the percentage of the entire population that has already had the virus. As of early May, the average in Spain was about 5 percent, though it varied substantially by region. From the study:

The findings from this nationwide seroprevalence study for SARS-CoV-2 indicate that the prevalence of IgG antibodies against this coronavirus is around 5% in Spain. Because the study was designed to obtain representative data at both national and provincial level, we were able to observe marked regional differences between the centre of Spain and the outskirts that generally match the surveillance data.

The prevalence in hotspot areas such as Madrid is more than five times higher than that observed in low-risk regions such as most provinces along the coasts.

To our knowledge, ENE-COVID is the largest population-based SARS-CoV-2 seroprevalence study in Europe. With more than 61 000 participants, the size of this study surpasses the combined 35 784 individuals described in a recent review of serosurveys…

One in three infections seems to be asymptomatic, while a substantial number of symptomatic cases remained untested. Despite the high impact of COVID-19 in Spain, prevalence estimates remain low and are clearly insufficient to provide herd immunity.

This graph above shows how the virus tends to be clustered around major cities and is far less prevalent in more rural areas. Here’s a closer view:

In addition to the study itself, Lancet published a note explaining its significance. Their conclusion is that any attempt to reach herd immunity (without a vaccine) would be “unethical.

In light of these findings, any proposed approach to achieve herd immunity through natural infection is not only highly unethical, but also unachievable. With a large majority of the population being infection naive, virus circulation can quickly return to early pandemic dimensions in a second wave once measures are lifted. In addition, the geographical variability and the dynamic of weekly increasing seroprevalence rates during the early phase of the pandemic highlight that these studies are only snapshots in time and space, and reflect the circumstances of the period in which they were done. As we are still in the midst of an unprecedented global health crisis, such seroprevalence data will continue to be necessary for public health authorities to estimate exposure rates, especially in areas with little testing capacity for acute cases. If and when a vaccine is widely available, ongoing seroprevalence studies will be able to provide information about the extent and duration of vaccine-induced herd immunity.

The minimum estimate for herd immunity is probably 60 percent. So obviously, as of early May, even the hardest hit cities in one of the hardest hit countries in Europe weren’t anywhere close. Bottom line: We could be dealing with this for a very long time still unless we find a vaccine.

Finally, a doctor named Ashish Jha used the data from the study to calculate an infection fatality rate of just over 1 percent.