Table 1.
Characteristics of the sample and incidence of reinfectionsa
| Overall sample | |
|---|---|
| (n = 7173) | |
| Mean age in years (SD) | 46.3 (21.8) |
| Age-class in years, % (n) | |
| < 18 | 11.6 (832) |
| 18–59 | 61.0 (4378) |
| ≥60 | 27.4 (1963) |
| Male gender, % (n) | 48.0 (3444) |
| Comorbidities or risk factors, % (n) b | |
| Diabetes | 5.7 (406) |
| Hypertension | 12.4 (891) |
| Major cardiovascular diseases | 6.4 (460) |
| COPD | 3.2 (231) |
| Kidney disease | 2.1 (149) |
| Cancer | 4.2 (301) |
| At least one of the above | 20.6 (1478) |
| First dose of SARS-CoV-2 vaccine c | 24.9 (1783) |
| Outcomes | |
| Reinfection, % (n) | 0.33 (24) |
| Reinfection and hospitalization, % (n) | 0.06 (4) |
| Hospitalizations after the second infection, % (n/N) | 16.7 (4/24) |
| Reinfection and deaths, % (n) | 0.01 (1) |
| Follow-up, mean (SD) | |
| Days from resolution to the end of follow-up, without reinfection (n = 7149) | 181 (90) |
| Days from resolution to reinfection (n = 24) | 201 (61) |
| % of reinfections by time after first infection resolution (n/N) | |
| 3–6 months after resolution | 0.13 (7/5510) |
| 6–9 months after resolution | 2.22 (13/585) |
| 9–12 months after resolution | 0.81 (4/495) |
| ≥12 months after resolution | 0.00 (0/918) |
SD = Standard deviation; n/N = Number of subjects with the outcome/Total number of subjects.
aA reinfection was defined as a new positive PCR test occurring ≥90 days after complete resolution of the first infection, and with at least two consecutive negative test results between episodes.
bSubjects with the selected comorbidities/risk factors in the Regional co-pay exemption database (Italian ‘Esenzioni Ticket’ file) or an hospital admission in the last ten years (from the Italian SDO database of administrative discharge abstracts) with the following ICD-9-CM codes in any diagnosis field: 250.xx (diabetes); 401.xx-405.xx (hypertension); 410.xx-412.xx or 414.xx-415.xx or 428.xx or 433.xx-436.xx (major cardiovascular or cerebrovascular diseases); 491.xx-493.xx (chronic obstructive pulmonary diseases—COPD); 580.xx-589.xx (kidney diseases); 140.xx-172.xx or 174.xx-208.xx (cancers).
cOf the 1783 participants who received the first vaccine dose during the follow-up, 1779 were immunized after the first positive nasopharyngeal swab; only four subjects were immunized before the first positive test.