COVID - 19 Transmission

Opening Remarks

  • Discussion of the COVID-19 pandemic and its uncertainties.

    • Current state: Uncertainty whether the pandemic is over, still a live area of ongoing research.

    • Connection to lived experiences and memories related to the pandemic.

    • University resources for support: Students encouraged to approach Student Support or tutors.

    • Disclaimer: Views are personal and do not reflect Welsh Government policy.

Overview of Coronaviruses

  • Different types of human coronaviruses:

    • Human coronavirus 229E (HCoV-229E)

    • Human coronavirus OC43 (HCoV-OC43)

    • Human coronavirus NL63 (HCoV-NL63)

    • Human coronavirus HKU1 (HCoV-HKU1)

    • Middle East respiratory syndrome-related coronavirus (MERS-CoV)

    • Severe acute respiratory syndrome coronavirus 1 (SARS-CoV-1)

    • Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2 or CoV2)

Transmission of COVID-19

Transmission Pathways

  • Short-range transmission:

    • Droplet transmission: Larger droplets fall quickly to the ground.

    • Aerosol transmission: Smaller particles can linger in the air longer.

    • Direct contact: Infected person to another through physical touch.

    • Indirect contact (fomite transmission): Virus transfer through contaminated surfaces.

  • Long-range transmission:

    • Aerosols: Can travel farther distances (>1 μm).

    • Indirect contact (fomite): Risk of transmission also exists.

Key Concepts in COVID-19 Transmission

Superspreading Events

  • Definition: Superspreading events significantly define the COVID-19 pandemic.

    • Statistics: Just 2% of SARS-CoV-2 positive individuals are responsible for 90% of virus spread in communities (Yang et al).

    • Condition for superspreading: High viral load, crowded environments, and poor ventilation.

Basic Reproduction Number (R)

  • R: Represents the average number of secondary infections produced by one infected individual.

    • Example: R = 3 indicating three new infections per case.

    • Importance of k: Refers to overdispersion in case clusters, illustrating variability in spread.

  • Due to super spreaders, means some where more likely to spread the droplets do to airway anatomy differences, covid was described as an overspread virus with no consistent R number

Asymptomatic Spread

  • Findings from studies (Poletti et al. 2020):

    • 80% of people <20 years and 74% <40 years showed no symptoms.

    • 40%-45% of all ages infected individuals may remain asymptomatic.

    • Asymptomatic individuals can still spread the virus (40-83% of cases are 'silent').

    • Viral load is typically highest around the time of symptom onset.

Transmission Mechanisms and Definitions

Terminology:

  • Ballistic droplets: Settle quickly, do not travel far.

  • Droplet nuclei: Smaller droplets that dry and become aerosols.

  • Aerosols: Suspensions of particles in gas, can float and travel distances longer than droplets.

Virus Dynamics in Aerosols

  • Respiratory secretions can carry virus particles which are affected by physical forces once outside of the host:

    • Evaporation and gravity impact virus survival outdoors.

Infectiousness of particles after disposition

  • Different activities will affect transmission of the virus

  • Individual anatomy affects the transmission of the virus

Mitigating Transmission Risks

Strategies to Reduce Close Range Aerosol Transmission

  1. Distancing: Reduces exposure and dilutes aerosol concentration.

  2. Masks: Critical for source control, especially in crowded settings.

Research on Masks

  • Masks proven to have a population-level effect on reducing transmission.

  • Importance of fit and performance:

    • High-performance masks like FFP2/N95 recommended for better source control.

Personal Space and Ventilation

Environmental Controls

  • Importance of ventilation to dilute aerosols and reduce transmission risk.

    • Practical advice: Open windows periodically to improve airflow.

Empirical Data on Air Quality and Transmissions

  • Use of CO2 levels as proxy metrics for evaluating infection risk indoors to estimate second hand air in a room :

    • Estimated rebreathed fraction critical for understanding indoor air quality.

Take Home Messages

  • Key factors in understanding COVID-19 transmission:

    1. Transmission is driven by outliers rather than the average individual.

    2. Superspreading events have a high association with viral load and crowded settings.

    3. Both close-range and long-range aerosol transmission exist and are critical.

    4. Masks, ventilation, and hand hygiene interact to lower transmission rates.

    5. Understanding and navigating COVID-19 now requires practical adaptations in behavior.