Increased transmissibility of the alpha SARS-CoV-2 variant: evidence from contact tracing data in Oslo, January to February 2021

Figures & data

Figure 1. Frequency distributions of the number of close contacts and secondary infections of the 415 index cases. (A) household close contacts; (B) secondary infections within households; (C) total close contacts; (D) total secondary infections. Index cases with zero close contacts were included in the estimation of reproduction numbers.

Table 1. The number of primary cases and their average number of close contacts and secondary infections by age and lineage.

	Alpha	Other lineages
Number of primary cases per age group
0–19 years	14	28
20–39 years	68	124
40–59 years	47	73
60+ years	16	44
Total	146	269
Mean number of close contacts within households (sd)
0–19 years	2.79 (1.63)	2.70 (1.56)
20–39 years	1.03 (1.36)	1.38 (1.60)
40–59 years	1.72 (1.87)	1.45 (1.43)
60+ years	1.25 (1.65)	0.34 (0.64)
All ages	1.45 (1.67)	1.36 (1.55)
Mean number of secondary infections within households (sd)
0–19 years	0.86 (0.95)	0.63 (1.15)
20–39 years	0.31 (0.63)	0.31 (0.73)
40–59 years	1.09 (1.48)	0.48 (0.75)
60+ years	0.38 (0.81)	0.11 (0.32)
All ages	0.61 (1.07)	0.36 (0.75)
Mean number of close contacts (sd)
0–19 years	19.64 (26.01)	15.00 (22.33)
20–39 years	5.30 (7.22)	7.01 (18.86)
40–59 years	6.09 (6.72)	4.71 (5.09)
60+ years	4.38 (4.86)	2.00 (2.56)
All ages	6.83 (10.94)	6.40 (15.28)
Mean number of secondary infections (sd)
0–19 years	1.07 (1.27)	1.36 (1.54)
20–39 years	0.61 (0.77)	0.73 (1.33)
40–59 years	1.51 (1.63)	0.90 (1.42)
60+ years	1.25 (1.61)	0.59 (0.95)
All ages	1.01 (1.30)	0.82 (1.34)

Table 2. The estimated secondary attack rates (SAR) for the alpha lineage and for other lineages, with the relative risk between them, and univariate analyses of the impact of age of primary case and household size on SAR within households for the alpha lineageand other lineages.

	Alpha	95% CI	Other	95% CI	RR	95% CI
Household SAR	0.42	(0.34–0.52)	0.27	(0.21–0.32)	1.60	(1.20–2.14)
- age-adjusted	0.36	(0.27–0.47)	0.28	(0.20–0.36)	1.51	(1.13–2.03)
Total SAR	0.15	(0.13–0.17)	0.13	(0.11–0.15)	1.16	(0.94–1.43)
- age-adjusted	0.15	(0.12–0.19)	0.15	(0.12–0.17)	1.21	(0.95–1.53)
Within household
Age of primary case
0–19 years	Ref		Ref
20–39 years	0.99	(0.49–2.07)	0.97	(0.56–1.76)
40–59 years	1.97	(1.08–3.87)	1.42	(0.81–2.60)
60+ years	0.98	(0.34–2.51)	1.43	(0.47–3.62)
Household size
1 contact	Ref		Ref
2 contacts	0.91	(0.44–1.95)	0.82	(0.45–1.46)
3+ contacts	1.00	(0.56–1.96)	0.68	(0.43–1.12)

Table 3. Estimated reproduction numbers for the alpha lineage and for other lineages, as well as the relative ratio between them.

Reproduction number	Estimate	95%CI
R other lineages	0.82	(0.72–0.93)
R alpha lineage	1.01	(0.86–1.19)
Ratio	1.24	(1.00–1.52)