Incubation period and serial interval of Covid-19 in a chain of infections in Bahia Blanca (Argentina)
Authors: Viego Valentina , Geri Milva , Castiglia Juan , Jouglard Ezequiel 
Published: June 2020

Objective

To estimate the incubation period and the serial interval of Covid-19 from a sample of symptomatic patients in Bahia Blanca city.

Methods

We collected dates of illness onset for primary cases (infectors) and secondary cases (infectees) for the first 18 secondary patients infected with SARS-Cov-2 in Bahia Blanca (Argentina). We ranked the fiability of the data depending upon certainty about the identification of the infector and the date of exposition to infector.

The sample has some missing values. In the case of incubation, as 3 patients were infected by other household members, we only have 15 observations with an observed date of exposition. For the estimation of serial interval, one patient became ill from close contact with an asymptomatic infectious. Also, estimations of both the incubation period and the serial interval were carried using the full sample and a subsample with higher certainty about the transmissor and date of exposition. By the time the dataset was prepared all infectors were recovered so estimations do need to take into account right censoring.

Results

The mean incubation period for symptomatic patients is 7.9 days (95% CI: 4.6, 11.1) considering the sample of 15 cases patients and 7.5 days (95% CI: 4.1, 10.9) if the sample is restricted to the most certain cases (n=12). The median is 6.1 (95% CI: 4.1, 9.2) and 5.8 (95% CI: 3.6, 9.3) respectively. Moreover, 97.5% of symptomatic cases will develop symptoms afert 13.6 days from exposition (95% CI 10.7, 16.5).

The point estimation for the mean serial interval is 6.8 days (95% CI: 4.0-9.6). Considering only the most certain pairs, the mean serial interval is estimated at 5.5 days (95% CI: 2.8, 8.1). The estimated median serial intervals were 5.2 (95% CI: 3.0, 8.1) and 4.1 (95% CI: 2.0, 6.9) days respectively.

Conclusions

Evidence from Bahia Blanca (Argentina) suggests that the median and mean serial interval of Covid-19 is shorter than the incubation period. This suggests that a pre-symptomatic transmission is not negligible. Comparisons with foreign estimates show that incubation period and serial interval could be longer in Bahia Blanca city than in other regions. That poses a signal of opportunity to attain more timely contact tracing and effective isolation.

Highlights

We estimate the incubation period in a sample of 15 symptomatic patients with Covid-19 in Bahia Blanca city (Argentina).

We estimate the serial interval for Covid-19 infections in a sample of 17 infector-infectee pairs detected in Bahia Blanca city (Argentina).

The median serial interval is lower to the median incubation period, suggesting a transmission is taking place also during the pre-symptomatic phase.

The incubation period and serial interval of Covid-19 in Bahía Blanca city seem to take more days than in Asian regions. This finding slows down the pace of health assistance to patients (conditional to public interventions).

Longer serial intervals help in tracing contacts and show relative slow turnover of case generations. At the same time, if symptoms take longer time to emerge, long serial intervals may also increase the reproductive number if contact tracing and effective isolation measures are placed untimely.