CDC Nowcast - The Dominance of Delta
#16,138
In a little over 3 months the Delta variant (and its sub variants AY.3, AY.2 & AY.1) have gone from a mere blip on our surveillance radar to causing an estimated 98% of the COVID infections in the United States; obliterating Alpha's record of just over 70% set in the late spring.
Delta's surge is even more impressive when you consider it has occured during the summer months, which are typically less conducive to viral transmission, and that we see similar transmission patterns in Europe, Asia, and Africa as well.
We've seen numerous attempts to explain this enhanced transmissibility, with some research suggesting changes in the spike protein, Delta's greatly increased viral load, or a greater concentration of the virus in the nasopharynx - are behind its remarkable spread.
Admittedly, all of these may be contributors to Delta's dominance. But a preprint, published last week, raises yet another apparent `difference' with Delta; a longer window for pre-symptomatic transmission.
First a link and a brief excerpt from a Nature news feature published Aug 19th.
Delta’s rise is fuelled by rampant spread from people who feel fine
People infected with the Delta variant generally do not have COVID-19 symptoms until two days after they start shedding the coronavirus.Smriti Mallapaty
People infected with the Delta variant of SARS-CoV-2 are more likely to spread the virus before developing symptoms than are people infected with earlier versions, suggests a detailed analysis of an outbreak in Guangdong, China1.
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Cowling and his colleagues analysed exhaustive test data from 101 people in Guangdong who were infected with Delta between May and June this year, and data from those individuals’ close contacts. They found that, on average, people began having symptoms 5.8 days after infection with Delta — 1.8 days after they first tested positive for viral RNA. That left almost two days for individuals to shed viral RNA before they showed any sign of COVID-19.(Continue . . . )
Influenza is generally believed to be transmissible up to 24 hours before overt symptoms appear, and past studies have suggested a similar `window' for asymptomatic COVID transmission.
The original 2002-2003 SARS virus, by contrast, did not appear to transmit well until after symptoms appeared, making containment possible (see SARS And Remembrance).
With nearly twice as long to spread via pre-symptomatic individuals - along with the other attributes mentioned above - it is not surprising that Delta is spreading as aggressively as it is.
Today's paper estimates that nearly 3 out of every 4 Delta infections are transmitted by those not yet showing symptoms.
Transmission dynamics and epidemiological characteristics of Delta variant infections in China
Min Kang, Hualei Xin, Jun Yuan, Sheikh Taslim Ali, Zimian Liang, Jiayi Zhang, Ting Hu, Eric H. Y. Lau, Yingtao Zhang, Meng Zhang, Benjamin J. Cowling, Yan Li, Peng Wu
doi: https://doi.org/10.1101/2021.08.12.21261991
This article is a preprint and has not been peer-reviewed
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ABSTRACT
BackgroundThe Delta variant of SARS-CoV-2 has become predominant globally. We evaluated the transmission dynamics and epidemiological characteristics of the Delta variant in an outbreak in southern China.
MethodsData on confirmed cases and their close contacts were retrospectively collected from the outbreak that occurred in Guangdong, China in May-June 2021. Key epidemiological parameters, temporal trend of viral loads and secondary attack rates were estimated and compared between the Delta variant and the wild-type SARS-CoV-2 virus. We also evaluated the association of vaccination with viral load and transmission.
ResultsWe identified 167 patients infected with the Delta variant in the Guangdong outbreak. The mean estimates of the latent period and the incubation period were 4.0 days and 5.8 days, respectively. A relatively higher viral load was observed in Delta cases than in wild-type infections. The secondary attack rate among close contacts of Delta cases was 1.4%, and 73.9% (95% confidence interval: 67.2%, 81.3%) of the transmissions occurred before onset. Index cases without vaccination (OR: 2.84, 95% confidence interval: 1.19, 8.45) or with one dose of vaccination (OR: 6.02, 95% confidence interval: 2.45, 18.16) were more likely to transmit infection to their contacts than those who had received 2 doses of vaccination.
DiscussionPatients infected with the Delta variant had more rapid symptom onset. The shorter and time-varying serial interval should be accounted in estimation of reproductive numbers. The higher viral load and higher risk of pre-symptomatic transmission indicated the challenges in control of infections with the Delta variant.
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We estimated that the 73.9% of transmissions occurred pre-symptomatically for the Delta variant, which was higher than other variants (7, 20, 21), suggesting a higher transmission potential of Delta cases before detection which was further supported by the high viral loads at least 4 days before illness onset shown in our study. The high risk of transmission particularly before onset indicated the need to expand contact tracing to a wider group of contacts and perhaps to a longer time scale in order to control the epidemic caused by the Delta variant (7, 22).However, for areas with a high prevalence of COVID-19, complete contact tracing and quarantine outside the home may be infeasible as the number of contacts is always several folds the number of infections (10). Physical distancing such as self-isolation and home quarantine is more suitable in these areas. However, society-wide physical distancing measures might increase transmission risk at household settings (10, 20). Our study showed that the secondary attack rate (22.0%) among household close contacts of Delta cases was higher than the rate obtained in 2020 (12.4%) in the same location with wild-type infections (23).
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In conclusion, the Delta variant demonstrated a higher transmissibility compared to the wild type of SARS-CoV-2. An extension of contact tracing period to perhaps four days prior to symptom onset may be needed considering the high proportion of pre-symptomatic transmission and the high viral load before onset in infections with the Delta variant. Inactivated vaccines appeared to be effective in reducing transmission of Delta infections and a high vaccination coverage should be pursued to reduce the burden of COVID-19 pandemic.
Fully vaccinated individuals continue to enjoy significant protection against serious illness, hospitalization, and death from COVID - including the Delta variant - even if breakthrough infections are increasingly common.
While it is hoped that booster shots will help reduce breakthrough infections (once they become widely available), the need for other protective measures - including face masks and avoiding crowds - has not gone away.
SARS-CoV-2 continues to evolve, adapt, and mutate into new variants. Hopefully Delta will be the pinnacle of this pandemic's reign, and nothing more formidable follows.
But so far, COVID has shown surprising resilience, and we've seen no signs that it is going away anytime soon.
