Over the last 6 months, we've learned a lot about how SARS-CoV-2 spreads🦠

What does the evidence so far tell us about SARS-CoV-2 transmission dynamics, high-risk activities and environments? Thread 🧵 (1/n)
The risk of transmission is complex and multi-dimensional. It depends on many factors: contact pattern (duration, proximity, activity), individual factors, environment (i.e. outdoor, indoor) & socioeconomic factors (i.e. crowded housing, job insecurity). (2/n) Image
Contact pattern:
We now know that sustained close contact drives the majority of infections and clusters. For instance, close family/friend contacts and gatherings are a higher risk for transmission than market shopping or brief community encounters. (3/n)
Even in the same household being a spouse/partner, sleeping in the same room or sharing the same sleeping space, frequent daily contact with the index case, and dining in close proximity has been associated with increased risk of transmission. (4/n)
For non-household contacts, engaging in group activities such as dining together or board games have been found to be high risk for transmission. So, the risk increases with longer & frequent exposure, close proximity, # of contacts, and group activities especially dining (5/n)
Individual factors:
Many ppl either do not infect anyone or infect a single person, and a large number of secondary cases are caused by a small # of infected ppl. Although this also is related to other factors, individual variation in infectiousness plays a major role.(6/n) Image
When we look at the viral load dynamics & contact tracing studies, those who are infected are very infectious for a short window, likely 1-2 days before and 5 days following symptom onset. No transmission documented so far after the first week of symptom onset. (7/n) Image
While asymptomatic patients can transmit the virus to others, emerging evidence suggests that asymptomatic index cases transmit to fewer secondary cases. Attack rates are highly correlated with symptom severity (8/n)
(medrxiv.org/content/10.110…) and (medrxiv.org/content/10.110…)
Transmission is also affected by other host factors, including host defence mechanisms and age. For instance, given the same exposure, susceptibility to infection increases with age, higher in > 60yo compared to younger or middle-aged adults. (9/n)
Contact pattern also depends on the setting of the encounter. Contact tracing studies suggest an almost 20x higher risk of transmission indoors compared with outdoor environments. (10/n) (ft.com/content/2418ff…) Image
Prolonged indoor contact in a crowded and poorly ventilated environment increases the risk of transmission substantially. But decreasing occupancy and improving ventilation through opening windows/doors can lower the risk. (11/n) Image
Much worryingly the largest outbreaks from across the world are reported in long term care facilities such as nursing homes, homeless shelters, prisons, and meat-packing plants where many people spend several hours working, living together, and share communal spaces. (12/n)
The largest clusters of cases observed in the USA have all been associated with prisons or jails. In the largest meat packing plant in Germany, while the common point of potential contact was workplace, risk was higher for a single shared apartment, bedroom and carpool. (13/n)
Socioeconomic factors and racial/ethnic disparities:
Global figures suggest that COVID-19 pandemic is strongly shaped by structural inequities, adverse living and working conditions and structural racism that drive household and occupational risks. (14/n)
People in lower-paid occupations are often classified as essential workers who must work outside the home and may travel to work on public transport. These occupations often involve greater social mixing, exposure risk due to prolonged working hours and job insecurity. (15/n)
Households in socioeconomically deprived areas are more likely to be overcrowded, increasing the risk of transmission within the household. These disparities also shape the strong geographic heterogeneities observed in the burden of cases and deaths. (16/n)
PHE surveillance report shows that while the number of infections is increasing mainly in 20-29, 30-39 ages in England, SARS-CoV-2 is spreading most in highly deprived areas - where people are in poorly paid work and can't afford to isolate. (17/n) assets.publishing.service.gov.uk/government/upl… Image
In Madrid, 37 neighbourhoods are seeing the highest incidence, 4 x the Spanish average. Common factors: these areas are poorer, denser and have a high proportion of immigrant population. (18/n)

Previous research suggests that although social distancing during the 2009 H1N1 pandemic was effective in reducing infections, this was most pronounced in households w greater socioeconomic advantage. Similar findings are emerging for COVID-19. (19/n) (pnas.org/content/117/33…)
Covid-19 could now be endemic in some parts of England that combine severe deprivation, poor housing and large BAME communities, national lockdown in these parts of the north of England had little effect in reducing the level of infections (20/n) (theguardian.com/world/2020/sep…)
A real overlap in the causes of mortality and deprivation can be seen here. The age-standardised rate of deaths involving COVID-19 in the most deprived quintile was more than double (2.3 times higher) than in the least deprived quintile in Scotland. (21/n) nrscotland.gov.uk/files/statisti… Image
In summary:
The disproportionate impact of COVID-19 on households living in poverty, and the racial and ethnic disparities observed in many countries, emphasize the need to urgently update our definition of "vulnerable" populations for COVID-19 & address these inequities. (22/n)
These include social and income protection and support to ensure low paid, non-salaried and zero-hours contract workers can afford to follow isolation and quarantine recommendations, provision of protective equipment for workplaces and community settings. (23/n)
Early viral load peak in the disease course indicates that preventing onward transmission requires immediate self-isolation with symptom onset (for a min of 5 days). Messages should prioritise isolation practices, and policies should include supported isolation. (24/n)
There are many things that could be done within families to decrease transmission. We need to provide clear instructions, and means of support to enable those with symptoms/positive test and their contacts to isolate. (25/n) abc.net.au/news/2020-09-1… Image
Policymakers and health experts can help the public differentiate between lower-risk and higher-risk activities and environments and public health messages could convey a spectrum of risk to the public to support engagement in alternatives for safer interaction (26/n)
Avoid crowded indoor poorly ventilated environments. Spend more time outdoors. Maintain your distance (more is better but 2 metre is not a panacea). Improve ventilation: open windows/doors. Wear a mask indoors. Wash hands. (27/n) (vox.com/science-and-he…) Image
Public health strategies will be needed to mitigate transmission in nursing homes, prisons and jails, shelters, meat-packing plants such as personal protective equipment and routine testing to identify infected individuals early in the disease course. (28/n)
This thread is based on our new paper with @JuliaLMarcus Caroline Buckee and @aetiology

Accepted in CID - preprint version can be accessed here: (papers.ssrn.com/sol3/papers.cf…) Image
Our article is now published online.

We argue that SARS-CoV-2 transmission dynamics should inform policy decisions about mitigation strategies for targeted interventions.

academic.oup.com/cid/advance-ar… Image

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More from @mugecevik

24 Aug
There is a lot of concern/confusion about vaccine effectiveness against the delta variant. How effective are the vaccines against Delta & how to interpret real-world observational data? So much misinformation is being circulated, so this thread brings key data together. 🧵(1/n)
Vaccine efficacy measures the relative reduction in infection/disease for the vaccinated vs unvaccinated arm. For instance, a vaccine that eliminates all risk would have an efficacy of 100%. Efficacy of 50% means you have a 50% reduced risk compared to an unvaxxed person. (2/n) Image
All studies assessing the performance of vaccines against Delta are based on real-world data (vaccine effectiveness), which are influenced by variant transmissibility, human behaviour, and immunity status of the population, therefore they require careful interpretation. (3/n) Image
Read 25 tweets
7 Feb
There is a lot of confusion about the efficacy of AstraZeneca/ChAdOx1 vaccine against COVID19 due to B.1.351 / 501Y.V2 - summarising the results of phase 1b/2a double-blind randomized trial conducted in South Africa (based on @GovernmentZA press conference).🧵(1/6)
Adults aged 18-65 years without severe comorbidities and HIV were recruited. It was designed to show >60% efficacy against symptomatic disease, but because only 2000 participants were recruited with 42 total events, this analysis was not statistically powered. (2/6) Image
In total, 1749 participants were recruited, the population enrolled was young and generally healthy; the prevalence of hypertension, respiratory disease, and diabetes was low. Therefore, it was not designed to assess efficacy against severe disease. (3/6) Image
Read 7 tweets
11 Jan
Concerns about outdoor transmission risk seem to be trending again. What is the risk of transmission outdoors and should we be more worried about outdoors with the new more-transmissible variant? 🧵(1/n)
The risk of transmission is complex and multi-dimensional. It depends on many factors: contact pattern (duration, proximity, activity), individual factors, environment (e.g. outdoor, indoor), socioeconomic factors, and mitigation measures in place. (2/n)(gov.uk/government/pub…\) Image
Transmission is facilitated by close proximity, prolonged contact, and frequency of contacts. So, the longer the time you spend with an infected person and the larger the gathering, the higher the risk is. (3/n) (academic.oup.com/cid/advance-ar…\) Image
Read 26 tweets
29 Jul 20
Very excited to share our new preprint "#SARSCoV2 viral load dynamics, duration of viral shedding and infectiousness: a living systematic review and meta-analysis" #IDTwitter

A huge team effort screening and synthesising the accumulating evidence. 1/6

2/6 Main findings:
- In the upper respiratory tract (URT), mean duration of RNA shedding was 17 days (95% CI, 15.5-18.6), max 83d
- Shedding duration increases with age & severity of illness
- There were limited studies reporting shedding duration in sputum (n=7) and stool (n=13) ImageImage
3/6 - High SARS-CoV-2 viral loads in URT are detectable in the first week, and peak viral load occurs at day 0-5
- In contrast, peaks in SARS-1 and MERS occurred at days 10-14 and 7-10 of illness
- Pts with SARS-CoV-2 are likely to be most infectious day 0-5 (3/6) Image
Read 6 tweets
4 May 20
A lot of discussion recently about transmission dynamics, most of which are extrapolated from viral loads & estimates. What does contact tracing/community testing data tell us about actual probability of #COVID19 transmission(infection rate), high risk environments/age?
1/ 2147 close contacts of 157 #COVID19 cases were followed up: Overall infection rate was 6%, higher infection rate among friends (22%) and household (18%), and main risk factors include contact in household (13%), transport (11%), dining (7%).
html.rhhz.net/zhlxbx/028.htm (4/3/20) Image
2/ 445 close contacts of 10 #COVID19 cases were followed up, of those 54 (12%) developed symptoms, suggesting secondary attack rate of 0.45%, household attack rate of 10.5%. No other close contacts incl community members, HCWs were positive.
cdc.gov/mmwr/volumes/6… (6/3/20) Image
Read 23 tweets

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