The sixth wave of coronavirus in Spain, driven by the highly transmissible variant of omicron, has multiplied positive cases among unvaccinated and vaccinated people, with nearly 1.4 million infections reported since the beginning of January and thousands more others not reported. However, Covid-19 vaccines are proving very effective in protecting against severe infection. While determining the exact level of this protection is complex, partial data collected in Spain and extensive analyzes in the United Kingdom show that vaccinated people are at a much lower risk of being hospitalized or dying from the virus.

The most detailed information in Spain comes from Catalonia, where data is broken down by vaccination status. Analyzing deaths in this northeastern region between December 23 and January 12 per 100,000 population, the death rate among individuals aged 70 and 80 is five times higher for the unvaccinated. Something similar happens in other age groups. In the 70-79 population, 50 deaths per 100,000 people are recorded among the unvaccinated, against 10 per 100,000 among the vaccinated, i.e. five times less. Meanwhile, in the 60-69 demographic, the disparity repeats itself: the death rate for the unvaccinated is 15 per 100,000 people, while for the vaccinated it is 5.5 per 100,000. .

Catalan data also shows that vaccine protection helps prevent hospital and intensive care unit (ICU) admissions. For vaccinated people between the ages of 70 and 79, the likelihood of ending up in hospital is almost six times lower than for those who are not vaccinated. Meanwhile, the chances of being admitted to an intensive care unit for Covid-19 are up to 10 times lower for the vaccinated than for the unvaccinated.

The Spanish Ministry of Health also publishes hospitalization data across Spain broken down by vaccination status, although in this case it is based on estimates: to calculate the number of unvaccinated in each age group, they subtract the number vaccinated from the population in 2020. However, the target population will have since increased, so it is possible that the ministry’s calculations overestimate the incidence in the unvaccinated. While the element of overestimation may explain why this figure is much higher than in Catalonia, the Ministry’s figures lead to similar conclusions: for people aged 60 to 79, for example, hospital admissions are 18 times higher in the unvaccinated.

In other countries, hospital data lead to the same conclusions. In Italy, where the sixth wave follows a similar pattern to that of Spain, the gap between vaccinated and unvaccinated is evident and continues over time. The probability of hospitalization is 10 times higher for non-vaccinated people in the 60-79 age group, a figure also used in Switzerland.

Booster shots boost protection

In the UK, the Health Security Agency (HSA) analyzed over half a million omicron infections in the last weeks of 2021. In order to draw accurate conclusions when comparing vaccinated groups and unvaccinated, many factors influencing the likelihood of infection and admission were taken into account, such as age, sex and region of residence, as well as whether a person has already tested positive, traveled to other countries or suffered from health problems. The aim is to monitor the effectiveness of vaccines by measuring how much the risk of infection, hospitalization and death is reduced by vaccination.

The HSA results confirm that the vaccines continue to protect those who are fully immunized, especially against serious diseases, but they also note a loss of effectiveness over a period of several weeks. Shortly after receiving the second dose, the effectiveness of vaccines in preventing hospitalization is about 72%, but drops to 52% after six months. In this sense, the good news is that a booster shot reinforces the protection; for people over 65, for example, Covid-19 vaccines are 90% effective in preventing serious illness.

What happens with infections in the omicron wave?

The new variant is very capable of infecting vaccinated individuals, as British studies have shown: the efficacy of two doses against symptomatic infections could drop to almost zero after six months, whereas with the delta variant it remained at 40%. Even after the booster shot, the effectiveness of the vaccine against omicron is partial, with rates between 40% and 60%, again with worse figures than those recorded against delta (90%).

These results are adjusted for many variables. In fact, when baseline infection rates are compared for vaccinated and unvaccinated without considering variables, they are very similar or even worse for the vaccinated group than for those who received no injections. This could be the case if there are many more people among the unvaccinated who have natural immunity against previous infection and this may explain why some studies find what looks like negative vaccine efficacy, as noted in reports from the UK and a preliminary study in Denmark.

Something similar is observed in official data from Iceland: the 14-day incidence rate is higher among the vaccinated population who did not receive a booster than among the unvaccinated (5,600 cases per 100 000 inhabitants, against 4000 in the group of non-vaccinated). However, the lowest incidence is among Icelanders with three doses.

This coincides with the data from Catalonia: in November, the incidence was lower for the vaccinated than for the unvaccinated in all age groups. But since omicron’s arrival, among the under-50 population — a demographic that has received few boosters — infection rates appear to be higher among those vaccinated.