Seven studies were included (approximately 13,206,095 people having primary vaccination and approximately 18,349,359 people having re-vaccination). Of these, five were period studies, one was a prospective cohort study and one was a clinic-based retrospective study.

Primary vaccination.

Post-vaccinial encephalitis (13,206,095 vaccinees): for all age groups combined, the CR per million was 2.9 (95% CI: 2.0, 3.9), the MR per million was 0.8 (95% CI: 0.4, 1.5) and the CFR per hundred was 28.9 (95% CI: 15.4, 45.9). There were 11 deaths among 38 cases of post-vaccinial encephalitis. Infants aged younger than 1 year were at greatest risk for post-vaccinial encephalitis (the risk ratio was 2.80 compared with older vaccinees). One prospective study that surveyed physicians found higher complication rates in infants aged younger than 1 year (176 cases per million versus 6.8 cases per million overall).

Vaccinia necrosum (13,206,095 vaccinees): for all age groups combined, the CR per million was 1.0 (95% CI: 0.5, 1.7), the MR per million was 0.2 (95% CI: 0.02, 0.5) and the CFR per hundred was 15.4 (95% CI: 1.9, 45.4). There were 2 deaths among 13 cases of post-vaccinial encephalitis. Vaccinees aged 20 years or older were at greatest risk for vaccinia necrosum (5.3 cases per million). Most cases occurred in people with previously diagnosed haematological malignancy or impaired immunity.

Eczema vacciantum (13,206,095 vaccinees): for all age groups combined, the CR per million was 12.8 (95% CI: 10.9, 14.9), the MR per million was 0.0 (95% CI: 0.0, 0.3) and the CFR per hundred was 0.0 (95% CI: 0.0, 2.2). There were 0 deaths among 169 cases of eczema vacciantum. There were 3 deaths among 132 cases in non-vaccinees after contact with a vaccinee. There was no difference across age groups.

Generalised vaccinia (13,206,095 vaccinees): for all age groups combined, the CR per million was 39.9 (95% CI: 36.6, 43.5). There were 0 deaths among 527 cases. Infants aged younger than 1 year were at greatest risk for generalised vaccinia (103.3 cases per million; the risk ratio was 2.80 compared with older vaccinees). Within age groups, the highest rates were reported in studies that surveyed physicians.

Accidental infection (13,206,095 vaccinees): for all age groups combined, the CR per million was 64.9 (95% CI: 60.6, 69.4). There were 0 deaths among 857 cases of accidental infection.

Re-vaccination.

The summary risks were lower than for primary vaccination. There were 2 deaths among over 19.5 million vaccinees.

Post-vaccinial encephalitis: the risk was 26 times lower; the CR per million was 0.1 (95% CI: 0.01, 0.4).

Vaccinia necrosum: the risk was 1.5 times lower; the CR per million was 0.7 (95% CI: 0.3, 1.1). There were 2 deaths among 12 cases. The CFR per hundred was 16.7 (95% CI: 2.1, 48.4).

Eczema vacciantum: the risk was 12 times lower; the CR per million was 1.0 (95% CI: 0.6, 1.6).

Generalised vaccinia: the risk was 29 times lower; the CR per million was 1.4 (95% CI: 0.9, 2.0).

Accidental infection: the risk was 3.8 times lower; the CR per million was 16.9 (95% CI: 15.1, 18.9).