Why might Delta be more transmissible?

Biology/Immunity COVID Variants

A: In the words of virologist Dr. Angela Rasmussen, “people infected with Delta are shedding a s**t-ton more virus than with earlier variants.”

A newly released study has provided some clues to Delta’s transmissibility tricks by testing viral loads in quarantined close contacts of confirmed cases. Contacts were tested daily by PCR, and the course of infection over time was documented, including viral loads and how long it took from exposure to testing positive and having symptoms.

❓What did they find?

People infected with Delta tested positive more quickly following exposure compared to earlier variants, suggesting more rapid viral growth.

💥The viral loads (by PCR Ct) were 1000 TIMES HIGHER in Delta when first detected compared to earlier variants.

➡️ This combination of a reduced incubation period and shedding of TONS more virus likely explains Delta’s increased transmissibility.

For example, if people are shedding 1000X more virus, the probability that a close contact will be exposed to an infectious dose is much higher. It may take a shorter interaction to transmit. If people become contagious more quickly after exposure, they also simply have more opportunity to infect others.

❓Is there any chance the virus floats through the air more easily than previous variants?

This study included quite detailed contact tracing and genetic sequencing of each infection. They found that Delta was transmitted both via direct contact & “indirect” transmission such as being in the same restaurant or transport. There was no evidence of more long-distance aerosol or unexplained transmission compared to previous variants.

➡️ This is good news–The virus still must obey the laws of physics, so while there is MORE of it being expelled, it’s not suddenly flying across the room more easily than it did before. 🙏

❓What does this mean for the efficacy of vaccination? These higher viral loads make sense with what we are seeing so far for vaccine protection.

⬇️ Unvaccinated people have no defense against Delta. When they are exposed to high levels of virus, it’s easy for Delta to gain a foothold.

➡️ Partially vaccinated people have some protection but low levels of neutralizing antibodies. This makes it easier for Delta to overcome those antibodies just by sheer numbers. One dose still provides good protection against severe disease, but less against symptomatic infection.

⬆️ Fully vaccinated people have high levels of neutralizing antibodies AND mature memory B and T cell responses. It’s a lot harder for Delta to overwhelm these defenses by just throwing more virus at them.

❓What about “breakthrough” infections?

The high amounts of virus being produced may help explain some of the positive tests we are seeing in fully vaccinated people. Since no vaccine is perfect, a high enough dose of virus can overcome vaccine-induced defenses enough to cause a detectable infection. The good news is that vaccine-induced immunity is limiting disease severity in most cases.

❓So what should we do to avoid this Delta?

The best defense is more of what you already know:

💥Get fully vaccinated

💥Especially if Delta cases are rising in your area, add precautions when spending time with people outside your household (mask, avoid crowds, limit gatherings, ventilate, etc).


➡️ Delta may be more transmissible because there is a lot MORE of it, more quickly after exposure.

➡️ Vaccines still work against Delta, but 2 doses are important.

➡️ Taking other precautions (masks, distancing, limit crowds, stay outdoors, ventilation, hand washing, etc) will further reduce risk.

(This post is adapted from Dr. Angela Rasmussen’s awesome twitter thread summarizing the new study.)

Link to study

Link to Original FB Post