Table 1.
Reported Measles Importations, Cases, Clusters, Outbreaks, and Genotypes—United States, 2001–2019
| Measles Cases, Outbreaks, and Clusters | 2001–2004 | 2005–2008 | 2009–2012 | 2013–2016 | 2017–2019 | Total (2001–2019) |
|---|---|---|---|---|---|---|
| Measles cases | ||||||
| Total cases, no. | 253 | 304 | 410 | 1131 | 1775 | 3873 |
| International importations, no.a | 123 | 109 | 161 | 160 | 194 | 747 |
| Importations associated with outbreaks, no. (% of importations)b | 43 (35) | 23 (21) | 34 (21) | 46 (29) | 68 (35) | 214 (29) |
| Outbreak-related cases, no. (% of total cases) | 116 (46) | 189 (62) | 196 (48) | 947 (84) | 1599 (90) | 3047 (79) |
| Measles outbreaksc | ||||||
| Total outbreaks, no. | 18 | 18 | 32 | 42 | 48 | 158 |
| Outbreaks with known source, no. (% of total outbreaks) | 15 (83) | 14 (78) | 20 (63) | 28 (67) | 37 (77) | 114 (72) |
| Outbreak size, median (range), no. of cases | 4 (3–14) | 4.5 (3–34) | 4.5 (3–21) | 5.5 (3–383) | 6 (3–697) | 5 (3–697) |
| Outbreak duration, median (range), d | 20 (3–61) | 28 (4–79) | 19 (5–68) | 22.5 (4–121) | 23.5 (5–322) | 22.5 (3–322) |
| Measles clustersd | ||||||
| Cluster size, no. (%) | ||||||
| Isolated or single cases | 101 (74) | 91 (75) | 164 (74) | 120 (62) | 126 (63) | 602 (69) |
| 2-Case clusters | 18 (13) | 12 (10) | 25 (11) | 32 (16) | 25 (13) | 112 (13) |
| 3–5-Case outbreaks | 11 (8) | 10 (8) | 17 (8) | 21 (11) | 23 (12) | 82 (9) |
| 6–24-Case outbreaks | 7 (5) | 6 (5) | 15 (7) | 15 (8) | 20 (10) | 63 (7) |
| 25–49-Case outbreaks | 0 (0) | 2 (2) | 0 (0) | 3 (2) | 1 (1) | 6 (1) |
| ≥50-Case outbreaks | 0 (0) | 0 (0) | 0 (0) | 3 (2) | 4 (2) | 7 (1) |
| Cluster duration, no. (%) | ||||||
| 0 Generationse | 104 (76) | 95 (79) | 170 (77) | 126 (65) | 134 (67) | 629 (72) |
| 1 Generation | 20 (15) | 9 (7) | 22 (10) | 23 (12) | 20 (10) | 94 (11) |
| 2 Generations | 4 (3) | 7 (6) | 19 (9) | 24 (12) | 23 (12) | 77 (9) |
| 3–5 Generations | 8 (6) | 8 (7) | 8 (4) | 16 (8) | 14 (7) | 54 (6) |
| ≥6 Generations | 1 (1) | 2 (2) | 2 (1) | 5 (3) | 8 (4) | 18 (2) |
| Genotyped, no. (%) | ||||||
| Isolated or single cases | 16 (16) | 24 (26) | 66 (40) | 62 (52) | 91 (72) | 259 (43) |
| 2-Case clusters | 6 (33) | 7 (58) | 17 (68) | 26 (81) | 22 (88) | 78 (70) |
| 3–5-Case outbreaks | 5 (45) | 5 (50) | 15 (88) | 20 (95) | 22 (96) | 67 (82) |
| ≥6-Case outbreaks | 6 (86) | 8 (100) | 14 (93) | 19 (90) | 25 (100) | 72 (95) |
| All measles clusters | 33 (24) | 44 (36) | 112 (51) | 127 (66) | 160 (80) | 476 (55) |
| Genotypes identified | B3, D3, D4, D5, D6, D7, D8, H1, H2 | B3, D4, D5, D6, D8, D9, H1 | B3, D4, D8, D9, G3, H1 | B3, D4, D8, D9, H1 | B3, D4, D8, H1 | B3, D3, D4, D5, D6, D7, D8, D9, G3, H1, H2 |
International importations are those arising in persons who acquired measles outside the United States and brought their infection into the United States.
Includes outbreaks associated with >1 importation; 36 (23%) of 158 outbreaks were associated with >1 importation (median, 3 importations per outbreak; range 2–13).
An outbreak is defined as ≥3 epidemiologically linked cases
Measles infection clusters include isolated or single cases, 2-case clusters, and outbreak of ≥3 cases.
Single cases were assigned a transmission duration of 0 days. Transmissions lasting ≤6, 7–14, or 15–24 days were considered to have 0, 1, or 2 generations of spread, respectively; subsequent generations were added every 10 additional days [9].