The success of Covid-19 vaccines against omicron: Vaccinated up to five times less likely to be hospitalized

Spain’s sixth coronavirus wave, driven by the highly transmissible omicron variant, has multiplied positive cases among both unvaccinated and vaccinated people, with almost 1.4 million infections reported since the beginning of January and thousands more going unreported. However, Covid-19 vaccines are proving highly effective in protecting against severe infection. While pinpointing the exact level of this protection is complex, partial data collected in Spain and extensive analysis in the United Kingdom show that vaccinated individuals are at a much lower risk of being hospitalized or dying from the virus.

The most detailed information in Spain has come out of Catalonia, where the data is broken down by vaccination status. Analyzing the deaths in this northeastern region between December 23 and January 12 per 100,000 inhabitants, the mortality rate among individuals aged 70 and 80 is five times higher for the unvaccinated. Something similar occurs in other age demographics. In the 70-79 population, 50 deaths per 100,000 people are reported among the unvaccinated, compared to 10 per 100,000 among the vaccinated, i.e. five times less. Meanwhile, in the 60-69 demographic, the disparity is repeated: the mortality rate among the unvaccinated is 15 per 100,000 people while, for the vaccinated, it is 5.5 per 100,000.

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

The Spanish Health Ministry also publishes hospitalization data throughout 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 of vaccinated from the population in 2020. However, the target population will have grown since then, so it is possible that the ministry’s calculations overestimate the incidence among the unvaccinated. While the element of overestimation may explain why this figure is much higher than in Catalonia, the ministry’s figures yield similar conclusions: for people between 60 and 79, for example, hospital admission is 18 times higher among the unvaccinated.

In other countries, hospital data leads to the same conclusions. In Italy, where the sixth wave is following a similar pattern to Spain, the gap between vaccinated and unvaccinated is evident and maintained over time. The probability of hospital admission is 10 times higher for the unvaccinated in the 60-79 age group, a figure also echoed in Switzerland.

Booster shots strengthen protection

In the UK, the Health Security Agency (HSA) has analyzed more than half a million omicron infections in the last weeks of 2021. In order to draw accurate conclusions when comparing vaccinated and unvaccinated groups, many factors influencing the probability of infection and admission were taken into account, such as age, gender and region of residence as well as whether a person had previously tested positive, traveled to other countries or suffered from any health conditions. The aim is to monitor the effectiveness of vaccines by measuring the extent to which 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 vaccinated, particularly against severe disease, but they also note a loss of effectiveness over a period of weeks. Soon after receiving the second dose, the effectiveness of the vaccines with regard to preventing hospital admission is around 72%, but this drops to 52% after six months. In this sense, the good news is that a booster shot strengthens protection; for those over 65, for example, Covid-19 vaccines are 90% effective at preventing severe disease.

What is happening with infections in the omicron wave?

The new variant is very capable of infecting vaccinated individuals, as studies from the UK have shown: the effectiveness 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, vaccine effectiveness against omicron is partial, with rates of between 40% and 60%, again with figures worse than those recorded against delta (90%).

These results are adjusted according to many variables. In fact, when the basic infection rates are compared for the vaccinated and unvaccinated without factoring in the variables, they are very similar or even worse for the vaccinated group than for those who have not received any shots. This could be the case if there are many more individuals among the unvaccinated who have natural immunity from a previous infection and may explain why some studies find what looks like negative vaccine effectiveness, as has been noted in reports from the UK and a preliminary study in Denmark.

Something similar is observed in the official data coming out of Iceland: the 14-day incidence rate is higher among the vaccinated population who have not received a booster shot than among the unvaccinated (5,600 cases per 100,000 inhabitants, compared to 4,000 in the unvaccinated group). However, the lowest incidence is among Icelanders with three doses.

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