Shot of a sickly senior woman blowing her nose with a tissue while sitting on a sofa ta home

A new study in more than 17,000 nursing home residents has found that a booster vaccination plus prior COVID-19 infection increased protection against severe COVID-19 outcomes in this population. This so-called hybrid immunity waned over time, however, in part due to the effect of new SARS-CoV-2 variants.

Investigators conducted a retrospective study of patients aged 60 and older living in nursing homes in the Basque area of Spain. Data came from an electronic health database and the country’s public health service records. Patients were vaccinated with the first generation mRNA vaccines and monitored until either their death or the study’s end, Jan. 31, 2022.

Whether they had received one, two or three shots, residents who had been infected with COVID-19 prior to being vaccinated had the strongest protection against severe COVID outcomes, the authors noted. “We found this hybrid immunity even after a single dose [of COVID-19 vaccine],” they wrote. 

Although natural infection and vaccination were protective against severe disease, they were only modestly effective in preventing transmission, or spread, between residents. The researchers attributed this finding to the emergence of the omicron variant near the study’s end. Omicron has been found to be more infectious and transmissible than prior variants. The relaxation of COVID-19 prevention measures and subsequent changes in behavior of vaccinated people may have also helped the disease to spread in these nursing facility settings, they added.

The results have been reflected in other studies, such as a review and meta-analysis published in the Lancet in February. That analysis compared the risk of COVID-19 in people with and without a prior infection up to September 2022. Patients with prior infection had very high (85%) lasting protection from reinfection by pre-omicron variants.

The authors of the current study said the link between prior infection, vaccination  and reduced disease severity show “a clear hybrid immunity effect.” 

In addition, “the finding that protection conferred by mRNA vaccines waned in the months after receipt of a third dose reinforces the importance of further consideration of additional doses to optimize protective immunity,” they concluded.

Full findings were published in JAMDA.

Related articles:

Hybrid COVID immunity conveyed by infection, shots lasts up to 12 months: study

Natural immunity from COVID-19 infection wanes over time: study

CDC: Seniors receive 80% protection against severe omicron illness with fourth vaccine dose