COVID-19 infections are generally uncommon in healthcare workers and MS patients who have been previously infected or who are fully vaccinated. However, it should be noted that most studies were done prior to the emergence of the Omicron variant.

In a preprint of the Canadian RECOVER study (REinfection in COVID-19 Estimation of Risk), the estimated incidence of COVID-19 reinfection was 3.3 per 100 person-years (Racine et al. medRxiv preprint, February 10, 2022). The observational study included 569 healthcare workers (mean age 42 years) from Montreal-area hospitals and healthcare facilities who had previously tested positive for CoV-2. Vaccination became available during the study period and 92.3% received at least one dose. Overall, five reinfections were recorded. The researchers concluded that the risk of reinfection was rare, although the incidence was substantially higher than what has been reported in other studies.

In the prospective CovaXiMS study, the breakthrough infection rate for fully vaccinated MS patients (N=1705) was 1.5%; infection occurred a mean of 108 days after the second vaccination (Sormani et al. medRxiv preprint, December 27, 2021). Nine of 23 infections occurred in patients receiving ocrelizumab, 6 on dimethyl fumarate, four on fingolimod and one on teriflunomide. Two patients (1 ocrelizumab, 1 teriflunomide) required hospitalization. The probability of infection was associated with lower CoV-2 antibody levels four weeks after the second vaccine dose.

An Italian retrospective study looked at the rate of breakthrough infections in MS patients (N=19,641) who were fully vaccinated (Schiavetti et al. medRxiv preprint, January 31, 2022). Breakthrough infection was defined as a PCR-positive test >14 days after the second or third vaccination. A total of 68 infections were seen over a median 8-month period. The overall rate of infection was low for patients on most DMTs (0.57%); the hospitalization rate was 3.9% (vs. 11.9% pre-vaccination). Infection rates were higher for patients receiving ocrelizumab (2.0%) and fingolimod (1.62%); the hospitalization rate in the ocrelizumab group was also significantly higher (16.7% vs. 19.4% pre-vaccination).

In interpreting these studies, several issues should be noted. ‘Fully vaccinated’ was generally defined as two doses of mRNA vaccine or one dose of the J&J vaccine; a third dose in Canada is variously referred to as a booster or additional dose (www.canada.ca/en/public-health/services/publications/healthy-living/canadian-immunization-guide-part-4-active-vaccines/page-26-covid-19-vaccine.html). However, some countries now define a full vaccine course as three doses of an mRNA vaccine or two doses of the J&J vaccine; a fourth or fifth dose would be considered boosters. Also, the term ‘breakthrough infection’ has fallen into disfavour since it implies that vaccine efficacy is defined as the prevention of infection (rather than prevention of serious outcomes). As such, infection in a vaccinated individual may be viewed by some patients as vaccine failure.