Is the number of coronavirus cases still the best measure of the pandemic?

Last October, the cumulative incidence (CI), i.e. the number of coronavirus cases per 100,000 inhabitants – usually measured over a period of 14 days – became the thermometer used by Spanish citizens to measure the state of the pandemic. Before then, in the first few months of the pandemic, the data point was not considered reliable as only the most serious patients were being tested for Covid-19. As testing increased, epidemiologists began to pay more attention to this indicator, but it was not until the Spanish Health Ministry published its traffic light system to measure risk that Spaniards began to understand that a CI of 500 cases is cause for concern while 50 or under is a cause for hope.

On Friday, the 14-day cumulative number of cases per 100,000 inhabitants reached 152, above the 150-threshold considered to indicate a situation of “high risk.” But with most of the at-risk population vaccinated against Covid-19 in Spain, this figure cannot be read in the same way. The CI is just one of eight main indicators and around 20 secondary ones that are used in the traffic light system to assess the level of risk in a territory. In addition to the CI, the system also takes into account to what extent the pandemic is affecting the elderly population and the pressure on Spain’s hospitals. These indicators now are playing a more important role in reflecting the risk of the pandemic – and they are not rising by a lot or are, in fact, falling.

As the vaccination drive has progressed, the incidence rate no longer has the same meaning, because it does not translate to hospitalizations and deaths in the same way

Pedro Gullón, from the Spanish Epidemiology Society

Pedro Gullón, from the Spanish Epidemiology Society, believes that it was probably “a mistake” to only look at the CI. “Since May, when the capacity for detection was much better until the beginning of this year, it was a very good indicator, because it was very stable and it was very easy to compare certain moments and the rise in transmission,” he explains. “But as the vaccination drive has progressed, it no longer has the same meaning, because it does not translate to hospitalizations and deaths in the same way. Now it is more useful to look at other statistics, like the incidence according to age group, or the rate of hospital occupancy, positivity [the number of tests that come back positive from the total], or mortality.”

According to the figures from the European Center for Disease Control and Prevention (ECDC), Spain has the third-highest CI in the European Union, behind only Portugal and Cyprus. But it has the lowest 14-day mortality rate per 100,000 inhabitants of the large EU states. In Spain, this data point stands at 5.75, below the rate in France (8.57), Italy (7.88) and Germany (11.04), which has a CI that is 10 times lower than Spain’s. Alex Arenas, a physicist and data expert at Rovira i Virgili University, warns that the mortality rate can be misleading, given that there tend to be delays in the notifications of fatalities in Spain, which are later reflected in the data on excess mortality from the National Statistics Institute (INE).

Gullón has several explanations for these differences: Spain may have a greater capacity to detect new cases than other countries; many vulnerable people in the country have died from Covid-19, meaning the mortality rate is unlikely to spike; the vaccination strategy in Spain by age groups has been one of the most effective in Europe; and lastly the incidence rate has only recently started to rise and, as such, has not been reflected yet in the number of fatalities (given that it takes time for an infection to become life-threatening and lead to death). What’s more, the spike in cases has been detected among younger age groups, who are less likely to develop a serious case of Covid-19. “Probably a little of each one of these hypotheses is playing a role,” says Gullón.

It is thanks to the Covid-19 vaccination drive that this wave – if it can be called that – has not become like the previous ones, which saw a rise in deaths and greater pressure on hospitals. Spain is one of the countries that has most strictly prioritized groups most at-risk to Covid-19, with age the guiding criteria. In Germany, for example, the vaccination drive began with the over-80s and residence of care homes, but after that, there was no clear order, reports Elena G. Sevillano. For two months, family doctors in private clinics in Germany were immunizing patients as they saw fit, meaning that many young people without prior health conditions received a shot before the over-60s or essential workers. This could explain, at least in part, why Germany has a higher coronavirus mortality rate than Spain. Although as Ildefonso Hernández, from the Spanish Society of Public Health (Sespas), points out, any spike in fatalities in Spain will take time to be reflected in the data.

Spain is one of the countries that has most strictly prioritized groups most at-risk to Covid-19, with age the guiding criteria

What is clear is that primary healthcare centers in Spain are under more pressure due to the rise in cases, although this is not measured in the Health Ministry’s alert system. The care of youngsters who contract the coronavirus, and normally have a mild or asymptomatic case, falls to these centers, which are also partly responsible for the Covid-19 immunization drive.

The experts who spoke to EL PAÍS agree that this new uptick in cases cannot be tackled the same way as previous waves. After a year and a half of restrictions, which have devastated the country socially and economically, experts say it is time to look for a different strategy to curb contagions. “Now we have to implement very specific measures,” says Hernández. According to the Sespas spokesperson, Spain should carry out “mass testing around new cases, looking for hotspots, which could be in nighttime venues, which some cities have already shut down. They are specific limitations, but in some cases will have to be toughened.”

The increase in contagions among young people in Spain could also have other consequences: more cases of long Covid (due simply to the fact that there are more infections); greater likelihood of more contagious strains and vaccine-resistant variants; more pressure on primary healthcare centers and the possibility of the virus spreading to older age groups, regardless of whether they have been vaccinated. The vaccine reduces the chance of contracting the virus, but does not eliminate the risk of infection. Half of the 60-69 population has received only one dose, given that the Oxford-AstraZeneca vaccine – whose second shot is administered up to 12 weeks after the first – is primarily being used for this group. Some regions – which are responsible for the Covid-19 vaccination drive as well as containing the pandemic in their territories – have reduced this period to 10 weeks or less to ensure that the 60-69 population are protected against the delta variant of the coronavirus, first detected in India. Covid-19 vaccines are less effective against this strain than the alpha variant, first detected in England, if only one shot has been administered.

The upward trend of the CI could also affect the tourism industry in Spain, which is considered a high-risk destination in Europe. Alberto Infante, professor of international health at the Carlos III Health Institute, warns that this could deter visitors from coming and prompt governments to introduce new travel restrictions on Spain. The United Kingdom, for example, restricts travel to nations with a very high CI – a measure Germany has announced it is also considering.

English version by Melissa Kitson.