In this short communication we report viral load dynamics in healthcare workers. The dynamics are different between Delta, BA.1, and BA.2 variants, the latter having the highest viral load in the first 10 days since initial PCR (7-fold and 3-fold respectively). However, the infection duration does not appear to be significantly different between variants.
The viral load dynamics of BA.2 in particular could have implications for transmission resulting in adaptation of the quarantine rules and advises compared to the rules and advises for the previous variants like Delta. For example in the Netherlands, the current quarantine advise is shortened to 5 days, and 7 days for healthcare workers, with the requirement that one has to be asymptomatic for at least one day. Based on the results of our study this would mean that 5 days after initial PCR the viral load of a BA.2 infected healthcare worker is significantly higher (5.7 log10 copies/mL [Ct-value 21]) compared to BA.1 (4.8 log10 copies/mL [Ct-value 24]), which is higher than for Delta (4.3 log10 copies/mL [Ct-value 26]) (table 3). After 7 days the viral loads are lower but for BA.2 a load of 4.4 log10 copies/mL (Ct-value 26) was detected, which is as high as Delta at day 5. Our result also show that at day 5 one still has a considerable load that is well above 3.2 log10 copies/mL (Ct-value 30).
The detected loads are most probably representing intact viral particles as samples with viral loads as low as 4.5 log10 copies/mL can be cultured (approximately 20%) of both vaccinated and unvaccinated individuals 7. Shamier et al. 2021 also demonstrated that on average the culture success is lower in sample from vaccinated individuals, likely because of presence of antibodies. By carefully extrapolating the data from vaccinated individuals this would mean that at one week since initial positive test about 25% of the healthcare workers, infected with BA.2 variant, would still have intact SARS-CoV-2 viral particles. Five days after initial positive test up to 45% of the BA.2 infected HCW and up to 25% of the BA.1 infected HCW would have intact viral particles. However, the culture results of Shamier et al. 2021 were based on variants that were dominant before the Omicron era 7. Since we know that in particular the Omicron variant is able to evade the immune system by escaping antibody responses, the presented interpreted percentages could even be higher. However, cellular tropism is shifted in the Omicron variant as it has impaired cell entry in lung and gut cells compared to Delta 8.
Apart from intact viral particles, the BA.2 variant might again lead to hospitalization as it is more infectious than the BA.1 variant 9. In addition, a previous BA.1 exposure does not appear to provide protection for a BA.2 infection 10. In January 2022 the BA.2 variant became the dominant SARS-CoV-2 variant in Denmark and together with high numbers of infections and lifted infection control measures, there was a surge of hospitalization. Therefore, the higher viral load dynamics of BA.2 compared to Delta and BA.1 should be considered in quarantine rules or advises, especially for healthcare workers who work with vulnerable patients.
One of the strengths of this study is the unique infection prevention strategy that was followed in our institution, allowing us to closely monitor infection progression per individual healthcare worker. Unfortunately we do not have information regarding vaccination status of the healthcare workers. However, we can assume most are vaccinated and received their booster based on anonymized data of our hospital. Also, we do not have data on symptoms per time point, thus association between viral load to symptoms is not possible nor the given advise to about length of quarantine. In addition, the number of cases with BA.2 is relatively small, but the viral load dynamics clearly differ, and the 95% confidence interval of the regression model is barely overlapping with Delta and BA.1 variants (figure 1).
In conclusion, viral load dynamics are different between the Delta, BA.1, and BA.2 variants with the latter having the highest viral loads, contributing to the higher transmission potential. This results in considerable viral loads even after the suggested quarantine period. As especially BA.2 has high loads after the advised quarantine period, that were shortened in the Netherlands, this could facilitate transmission of the virus. This highlights the need for a tailored approach per variant when it becomes dominant as results of previous variants do not always match. This tailored approach is especially important for healthcare workers as they can transmit SARS-CoV-2 to vulnerable patients in for example hematology or oncology wards.