A recent study looked at reinfection risks in long-term care and retirement facilities¹. The study explored the impact of previous SARS-CoV-2 infections and vaccination on the risk of reinfection with the Omicron variant among older adults living in long-term care and retirement homes. The researchers investigated whether hybrid immunity resulting from a combination of natural infection and vaccination provides protection against subsequent Omicron infections. The results of the study surprised the researchers. While hybrid immunity from a prior Omicron BA.1/2 infection initially appeared protective against the same variant, it was unexpectedly associated with an increased risk of Omicron BA.5 reinfection.

The study demonstrates there is a variant dependent difference in the immune response and that hybrid immunity is not necessarily universally beneficial against all variants. It highlights an urgent need for further research to better understand the dynamics of immune protection against evolving variants, especially in vulnerable populations like older adults in long-term care or retirement living facilities.

The Great Barrington Declaration, published in 2020², suggested that the vulnerable and elderly could be shielded from infection while the rest of the population built so-called ‘herd immunity’ but this study disproves the viability of such a strategy even against the backdrop of vaccination and infection of the vulnerable and elderly population. Hybrid immunity, the public health policy descendent of herd immunity, does not have uniformly protective effects.

A second recently published study examined evidence of leaky protection following COVID-19 vaccination and SARS-CoV-2 infection in an incarcerated population³. The study investigated the nature of SARS-CoV-2 immunity, particularly whether infection and vaccination offer all-or-nothing protection or leaky (exposure-dependent) protection against subsequent infection. The researchers utilized the controlled environment of correctional facilities to overcome challenges related to measuring viral dose and exposure risk. They investigated how exposure to infected individuals within different proximity settings (cell and cell block exposures) affected the risk of infection and the effectiveness of prior infection, vaccination, and so-called hybrid immunity (prior infection and vaccination combined).

The researchers found there was an elevated risk of infection associated with high-exposure settings (cells and cell blocks) compared to low-exposure or non-exposure events. The study showed that the effectiveness of immunity differs based on exposure settings, with hybrid immunity (prior infection and vaccination) generally providing the highest protection, followed by prior infection and vaccination alone, but that no form of immunity was wholly protective against high exposure events.

The study’s findings were consistent between periods dominated by the Delta and Omicron variants, emphasizing that exposure was of critical importance even before the emergence of a variant with greater capacity for immune escape.

This study suggests that inoculum dose, the term for the amount of virus a person is exposed to, plays an important role in the likelihood of infection even in the context of vaccination and prior infection, an issue that has been explored in relation to COVID-19 and other pathogens⁴. The results of the study suggest additional protective measures should be used in all high exposure settings.

Given the ongoing risk posed by the long-term post-acute sequelae of COVID-19, commonly known as Long COVID^5,6, every infection has the potential to be harmful, and even though infection acquired immunity and vaccination protect against severe outcomes during the acute phase of COVID-19⁷, they do not wholly protect against Long COVID⁸, which means each infection carries a risk that a person will develop symptomatic Long COVID. This might mean weeks of reduced physical health or years of debilitating symptoms⁵, a harmful risk people should not be exposed to in day-to-day life.

The long-term care and correctional facilities studies suggest a multi-layered approach to public health is essential and that a vaccine-only strategy is unsustainable long-term. There is growing evidence that government agencies recognize the necessity of a more comprehensive strategy, with a movement towards recommending multi-layered protections as the only way to avoid the risk of Long COVID⁹.

We would like to see governments communicate the risks to the general public more effectively and inform people how they can take steps to protect themselves while living their daily lives. At the very least, people should be properly informed, so they are not unwittingly exposing themselves to potentially life-altering risks.

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