'Alarming': Over-65s at much higher risk of Covid reinfection than young

Experts described the findings as "relatively alarming" given that previous research had painted a picture of reinfections as an "extremely rare event". 

The authors said it emphasised the importance of measures to protect the elderly, including physical distancing and vaccinations, even if they have previously fought off the virus. 

In Denmark testing was open to people with and without symptoms

Dr Steen Ethelberg, head of zoonotic infection at the Statens Serum Institut in Copenhagen, said: “Our study confirms what a number of others appeared to suggest: reinfection with Covid-19 is rare in younger, healthy people, but the elderly are at greater risk of catching it again.

"Since older people are also more likely to experience severe disease symptoms, and sadly die, our findings make clear how important it is to implement policies to protect the elderly during the pandemic.

"Given what is at stake, the results emphasise how important it is that people adhere to measures implemented to keep themselves and others safe, even if they have already had Covid-19.

"Our insights could also inform policies focused on wider vaccination strategies and the easing of lockdown restrictions.”

READ MORE: 'Up to one in four' Scots now have protective Covid antibodies

It comes as much of continental Europe is facing a third wave of coronavirus and the slow rollout of the immunisation programme has been plunged deeper into crisis with the suspension of the Oxford/AstraZeneca vaccine in several countries - including Denmark - over a blood clots scare. 

Regulators insist the vaccine is safe and there is no causal link. 

Cases are on the rise in Europe, including in Denmark, France, and Germany, which have suspended the AstraZeneca vaccine

To date, there have been more than 120 million known cases of Covid worldwide and nearly 2.7 million deaths.

However, the degree to which catching Covid confers protection against repeat infection has been poorly understood.

In other coronaviruses, such as those which cause the common cold, immunity wanes within around 11 months, but lasts two to three years in the case of the much deadlier coronaviruses, SARS or MERS.

In the first large-scale study of its kind, the Danish researchers analysed patient data from the first and second waves of Covid in Denmark during 2020.

READ MORE: Blood clot scare was last thing Europe's struggling vaccine rollout needs 

The country has one of the most comprehensive testing regimes in the world, with four million people - 69% of the population - taking a PCR test in 2020. 

The TestCenter Denmark system is free, available nationwide, and open to people with or without symptoms. 

Among the 11,068 people who had tested positive for Covid during the first wave, between March and May, 72 tested positive again during the second wave from September to December 2020 (a 0.65% reinfection rate).

The researchers adjusted their estimates for the immunity rate to into account factors such as increasing testing

Among younger Danes - the under-65s - there were 9,137 positive cases during the first wave and 55 cases of reinfection in the second wave (0.6%).

For those aged 65 and older, there were 1,931 positive cases during the first wave and 17 cases of reinfection during the second wave (0.88%).

Ratios of positive and negative test results were calculated taking account of differences in age, sex, and time since infection, and then adjusted for factors such as cumulative exposure risk and number of PCR tests.

From this, the researchers were able to extapolate that the protective immunity rate for the 65-plus age group was 47% compared to 80.5% for the under 65s.

READ MORE: 'No evidence whatsoever' that vaccines causing blood clots

The authors noted that the "natural age-related changes in the immune system of older adults" could explain why they were more vulnerable to reinfection, as well as more susceptible to serious illness as a result of the virus. 

Co-author Dr Daniela Michlmayr, a virologist at Staten Serum Institut, said their analysis "did not identify anything" to suggest that the level of immunity acquired from infection - even if it is only 47% - declines in the six months following recovery from Covid.

The time period of the study meant it was not possible to evaluate the risk of reinfection from some of the newer - more transmissible - Covid variants, such as the Kent, South African, and Brazil variants, which began spreading towards the end of 2020 and did not reach Denmark until 2021.

Danish Prime Minister Mette Frederiksen

The Brazil variant in particular was associated with a severe outbreak in the city of Manaus, where it originated, despite previously high infection rates in the local population.

READ MORE: Why Brazil's Covid variant turned herd immunity on its head

The authors noted that detailed clinical information is only available if patients are admitted to hospital, so it is not possible to say whether repeat infections are milder than the first - although this would be expected.

Professors Rosemary Boyton and Daniel Altmann, of Imperial College London, describe the findings as "relatively alarming".

In a comment article linked to the study, they write: “Set against the more formal reinfection case reports that are based on differential virus sequence data and make reinfection appear an extremely rare event, many will find the data reported [here] about protection through natural infection relatively alarming.

"Only 80% protection from reinfection in general, decreasing to 47% in people aged 65 years and older are more concerning figures than offered by previous studies."

They add: “These data are all confirmation, if it were needed, that for SARS-CoV-2 the hope of protective immunity through natural infections might not be within our reach and a global vaccination programme with high efficacy vaccines is the enduring solution.”