Table 1.
Characteristics of definitions of close contacts and infection and attack rates in four Asian countries.
| Time | Distance | Protective measure* | Observation period | Secondary infection rate | Secondary attack rate | ||
|---|---|---|---|---|---|---|---|
| <2.0 m | <1.0 m | ||||||
| Taiwan | 15 min or more (conversing) | ○ (medical institution) | None (medical institution) | Jan – Apr 2020 | 0.8% | 0.7% | |
| Singapore | 30 min (with patient) | ○ | - | Jan – Apr 2020 | - | 2.4% | |
| Guangzhou, China | ○ | None (social life) | Jan – Mar 2020 | 3.7% | 3.5% | ||
| Guangzhou, China | ○ | None (social life) | Before March 2020 | - | 4.4% | ||
| Hubei Province, China | ○ | None (social life) | Jan – Feb 2020 | - | 6.5% | ||
| Nagano, Japan | 15 min or more (contact) | ○ | None (social life) | Aug – Sep 2020 | - | 15.0% | |
| Our study | Had prolonged contact | ○ | None (social life) | Mar – Jun 2020 | Period 1:13.3% | - | |
| Fukui, Japan | 15 minutes or more (contact) | ○ | None (social life) | Jul – Sep 2020 Oct 2020 – Feb 2021 | Period 2:19.2% Period 3:20.0% | - - | |
Other conditions (which varied from country to country) included living together (in the same room), engaging in medical and welfare services, diet, vehicles, number of days of symptom onset in COVID-19 index patients, and contact with infectious agents.
Table 2.
Characteristics of the cases of SARS-CoV-2 infection in Fukui Prefecture, Japan.
| Period 1* (n = 122) | Period 2† (n = 122) | Period 3‡ (n = 301) | P | |
|---|---|---|---|---|
| Age (years;IQR) | 54 (45.3–66) | 59 (42–77) | 48 (28–66) | <0.0001§ |
| 0–10 | 2 (1.6) | 1 (0.8) | 12 (4) | 0.178 |
| 11–20 | 3 (2.5) | 6 (4.9) | 24 (8) | 0.081 |
| 21–30 | 7 (5.7) | 12 (4.9) | 55 (18.3) | 0.001|| |
| 31–40 | 14 (11.5) | 10 (8.2) | 38 (12.6) | 0.444 |
| 41–50 | 18 (14.8) | 12 (9.8) | 39 (13.0) | 0.536 |
| 51–60 | 36 (29.5) | 10 (8.2) | 43 (14.3) | <0.0001** |
| >61 | 42 (34.4) | 71 (58.2) | 90 (29.9) | <0.0001†† |
| Sex, n (%) | ||||
| Male | 74 (60.7) | 65 (53.3) | 145 (48.2) | – |
| Female | 48 (29.3) | 57 (46.7) | 156 (51.8) | – |
| Presence of underlying disease | 59 (48.4) | 53 (43.4) | 96 (31.9) | 0.005‡‡ |
| Unknown underlying disease | 0 (0) | 0 (0) | 17 (5.6) | – |
Fisher's exact test was used for pairwise comparisons of categorical variables. The Kruskal–Wallace test was used for pairwise comparisons of continuous variables. The Holm adjustment was used for multiple comparisons.
Table 3.
Number of close contacts and SARS-CoV-2 positivity rate among close contacts patients with COVID-19 in Fukui Prefecture, Japan.
| Period 1* | Period 2† | Period 3‡ | P | |
|---|---|---|---|---|
| Number of close contacts | 429 | 291 | 767 | – |
| Positivity rate among close contacts, n (%) | 57 (13.3) | 56 (19.2) | 154 (20.0) | 0.009 |
Fisher's exact test was used for pairwise comparisons of categorical variables. The Holm adjustment was used for multiple comparisons. * vs. †, P = 0.073; * vs. ‡, P = 0.010; † vs. ‡, P = 0.796
Cochran–Amitage test, a test of trend, was used to evaluate the increasing/decreasing trend in the percentage of infections among the close contacts in the first, second, and third periods.
Cochran–Amitage test P = 0.004
Table 4.
Odds ratios of having a positive SARs-CoV-2 test result among close contacts of patients with COVID-19.
| Period | Close contact with COVID-19 patient | SARS-CoV-2 test | Stratified independence test (Pearson's χ2 test) | Stratified odds ratio | ||
|---|---|---|---|---|---|---|
| Positive | Negative | Odds ratio | 95% confidence interval Lower-Upper | |||
| 1* | No, n (%) | 65 (2.2) | 2866 | P < 0.0001 | 6.76 | 4.66–9.80 |
| Yes, n (%) | 57 (13.3) | 372 | ||||
| 2† | No, n (%) | 66 (1.1) | 6011 | P < 0.0001 | 21.70 | 14.85–31.71 |
| Yes, n (%) | 56 (19.2) | 235 | ||||
| 3‡ | No, n (%) | 147 (0.7) | 21596 | P < 0.0001 | 36.90 | 29.04–46.91 |
| Yes, n (%) | 154 (20.0) | 613 | ||||