Did the incidence decrease after infection or vaccination?

Within the naïve group, approximately 26 new cases were reported per 100 person-years. In the second group, which consisted of previously infected and unvaccinated individuals, and the third group of vaccinated individuals, there were zero and 1.6 cases, respectively, per 100 person-years.

This implies an incident rate ratio (IRR) of zero following natural infection with the virus. That is, when the incidence among those with a history of infection (zero) is compared to that among the naïve individuals (26), the former have no risk of reinfection.

Conversely, the IRR was 0.06 when the incidence among vaccinated (1.6) and naïve (26) individuals were compared. Therefore, the risk of SARS-CoV-2 infection following vaccination was less than one-tenth of the latter. Finally, vaccine recipients had no increased risk of breakthrough infection with the virus, as compared to those with a history of prior infection.

What are the implications?

The current study demonstrates that both prior infection and receipt of an mRNA vaccine confer a high degree of immunity against reinfection or breakthrough infections, respectively. This observation has also been described in other stories that reported the equivalent and high protective efficacy of vaccination with the BNT162b2 or ChAdOx1 nCOV-19 vaccines and prior infection with the virus.

This protection likely operates through both humoral and cellular arms of the adaptive immune response. The virus S protein and other antigens elicit specific and durable T cell responses targeting these epitopes. The diversity of the T cell response allows a breadth of protection against the ancestral SARS-CoV-2, as well as key variants of concern such as the B.1.1.7, B.1.351, and P.1 lineages.

Moreover, memory B cells have been detected that provide strong antibody-mediated immunity over the long term, up to six months after infection.
Combined with earlier studies, our findings should provide increased confidence that those previously infected are at very low risk for repeat infection."
A possibility to be ruled out by further study is the rise of new variants, which could be an alternative explanation for the low detection of the virus in this group caused by mutations that prevent recognition of the viral genetic material by the RT-PCR primer sequences.

*Important Notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.
Journal reference:

• Kojima, N., Roshani, A., Brobeck, M., et al. (2021). Incidence of Severe Acute Respiratory Syndrome Coronavirus-2 infection among previously infected or vaccinated employees. medRxiv. doi:10.1101/2021.07.03.21259976.https://www.medrxiv.org/content/10.1....03.21259976v1

Though more research is needed, the findings add evidence that people who received mRNA COVID-19 vaccines may not need an additional “booster” shot for quite some time, unless SARS-CoV-2 evolves into new forms, or variants, that can evade this vaccine-induced immunity. That’s why it remains so critical that more Americans get vaccinated not only to protect themselves and their loved ones, but to help stop the virus’s spread in their communities and thereby reduce its ability to mutate.

The new study was conducted by an NIH-supported research team led by Jackson Turner, Jane O’Halloran, Rachel Presti, and Ali Ellebedy at Washington University School of Medicine, St. Louis. That work builds upon the group’s previous findings that people who survived COVID-19 had immune cells residing in their bone marrow for at least eight months after the infection that could recognize SARS-CoV-2 [2]. The researchers wanted to see if similar, persistent immunity existed in people who hadn’t come down with COVID-19 but who were immunized with an mRNA vaccine.