Table 1 shows some characteristics of participants in the two groups of SARS-CoV-2 patients (primary cases, n=48) and their contact cases (n=453) in groups of household and non-household. Forty-eight laboratory confirmed SARS-CoV-2 infections were detected in the only university hospital of Rafsanjan (local population = 320,000), (March and April 2020), among whom 9 (18.8%) were asymptomatic. Mean age of patients and contact cases were 53.0±18.2 (median = 50.5, Min=25, Max=95), and 30.77±19.3 (median = 31, Min=1, Max=91) years, respectively. All contact cases of 40 primary cases were followed for three months beginning from each SARS-CoV-2 patient laboratory diagnosis date. Contact tracing identified 453 contact cases including the two groups of 206 household and 247 non-household contact cases. Among primary cases (n=48), the first most common symptoms were cough (10, 21%), fever and chills (10, 21%). First symptom of SARS-CoV-2 in one primary case was eye irritation (1, 2%). Nine (18.8%) SARS-CoV-2 primary cases were asymptomatic (Table 2).
The number of primary cases contacts;
Our results showed that some contact cases were permanently living with a primary case. We counted such contact case as only one permanent contact per day. Contacts were recorded based on the three stages of ; 1- incubation period = 7 days, 2- illness period = 6.4 ≈ 7 days and 3- convalescent period = 7 days. Overall number of contacts during incubation period was 1348. Since, data of 63 contact cases was not, completely recorded, we divided the overall number of contacts to 34.5 instead of 40 primary cases. The mean number of contacts was 39.1 giving 5.6 (39.1/7 = 5.6 ≈6) daily contacts for each primary case during the incubation period. The number of daily contacts in the illness period and convalescent period were estimated similarly, applying the same method, giving 5.6 (≈6) and 5.4 (≈6) contacts per day, respectively.
SARS-CoV-2 household and non-household SARs;
Further, household and non-household SARs among contact cases along with 95% confidence interval are reported in table 3. Our results of serology analysis (IgG antibody) showed an overall attack rate of 15.3% (95%CI; 10.9 - 19.7). Our results demonstrated that the SAR increased when the age of contact cases increased (Table 4). Household and non-household SARs of SARS-CoV-2 infection were 20% (95%CI; 12.7 – 27.3) and 11.3% (95%CI; 6.1 - 16.5), respectively. The risk of infection transmission among household contact cases was 1.41 (OR = 1.41, 95%CI; 0.96-2.1) times higher than that risk among non-household contact cases.
The proportion of asymptomatic COVID 19 infections;
Among traced contact cases with serology results (n=257), 18 secondary cases were infected without any symptoms giving about 9.5% of contact cases who were asymptomatic. On the other hand, this group of secondary cases (n=18), consist 46% of all infected secondary cases.
Out of 257 participants with the results of serology analysis, 25 were exposed to asymptomatic primary cases of which 7 (28%, 95%CI; 10-46) were IgG positive. whereas, among 232 participants exposing to symptomatic primary cases, there were 32 (13.8%, 95%CI;9.4- 18.2) IgG positive participants. Transmission risk of SARS-CoV-2 among contact cases exposing to asymptomatic patients was significantly higher than that among contact cases exposing to symptomatic primary cases (OR = 2.3, CI: 1.01 – 4.11).
How long antibodies against SARS-CoV-2 infection may last?
Mean duration past from exposure (contact to primary cases) between the two groups of contact cases with (≥1) and without (<1) IgG antibodies (40.5±15.9 and 36.9±15.2, respectively) were not significantly different. There was also no significant correlation between the titer of IgG and the time past from the exposure among contact cases showing that antibodies against SARS-CoV-2 infection would remain for a long time in the serum of recovered people.
SARS-CoV-2 serial interval;
The time distance between the date of the first symptom of primary cases (SARS-CoV-2 patients) and the date of the first symptom of secondary cases was measured. Mean SARS-CoV-2 serial interval was 6.4±4.6 (95% CI; 5.21 – 7.6) days with a median of 5 days (Min=1, Max=17 days).
Comorbidity effect on SARS-CoV-2 transmission;
There was no higher risk of becoming infected by SARS-CoV-2 infection due to having comorbidity as the two proportions of people with IgG ≥1 among participants with and without comorbidity (13.7% and 15.5%, respectively) were not statistically different. However, among traced contact cases with serology results (n= 257), 19 were suffering from hypertension and were more probable to have positive results of IgG (31.6%, n=6) comparing to participants who did not reported this health problem (13.9%, 33 out of 238) (OR= 2.9, 95% CI; 1.02 – 8.7).
Novel coronavirus transmission probability per each contact;
In order to measure the probability of infection transmission per each contact (Pt), the number of contacts with primary cases among participants with serological analysis was registered. The number of contacts were recorded based on the three stages of SARS-CoV-2 patients infectiousness; 1- incubation period (Ninc) = 645 (7 days), 2- illness period (Nill)= 644 (6.4≈ 7 days) , and 3- convalescent period (Ncon) = 613 (7 days). Also, the number of infected secondary cases (Nsec)= 39.
Pt = Nsec / Ninc + Nill + Ncon
Pt = 39/645+644+613 = 0.0205
The overall probability of SARS-CoV-2 transmission per each contact is about 0.0205. The probability of SARS-CoV-2 transmission per each household and non-household contacts were 0.0161 and 0.0337, respectively.
The basic reproductive number of SARS-CoV-2 infection (R0);
Regarding aforementioned indices derived from our results, it was possible to fit suitable models to predict SARS-CoV-2 infection spread in human populations. We used statistical software of R to provide models.
R0 = P*C*D
Where, P, C and D stand for the probability of transmission per each contact, the number of contacts per unit time (day) and the duration of SARS-CoV-2 infectiousness, respectively.
Figure 2 shows, regression models illustrating the extend of which the value of R0 depends on the number of household and non-household daily contacts among contact cases. This model predicts the size of change in R0 value when the number of daily contacts to infected cases decreases in household and non-household contact cases.
Our data also demonstrated the size of reduction in R0 value according to decreasing the duration of symptoms (illness period) among infected cases (figure 3).