Jessica writes:

Hello Esteemed TWiV Hosts,

I’m curious if the problems in efficacy in the ChAdOx vaccine (AZ) in South Africa are related to seroprevalence of chimpanzee adenovirus neutralizing antibodies?  I feel like there has long been a presumption that chimp adenovirus would be a better vector because fewer people would have neutralizing antibodies in the general population, but to date I’ve only seen limited studies that evaluate whether or not this is the case.  

Considering that physicians don’t typically do anything about adenovirus infections chalking them up to be a common cold, wouldn’t it be possible that infection from a chimpanzee adenovirus would fly under the radar?  Perhaps in the case of the AZ vaccine, the vector is neutralized before the benefit can be achieved? 

I see a lot of media correlating the lack of efficacy on the composition of the variant but I feel like it could be possible that exposure to ChAd might be involved? 


Thank you for everything you do. I’m SO happy that so many more people know what I’m talking about when I say “my favorite podcast TWiV…”

Stay safe and well,


“I have the audacity to believe that peoples everywhere can have three meals a day for their bodies, education and culture for their minds, and dignity, equality and freedom for their spirits.”

-Dr. Martin Luther King Jr.

Lind writes:

Last night my husband and I facing yet another Saturday night in COVID 19 isolation began pondering the profound question posed by Rich:

Why DO wombats have cubic poop?

Husband, a physicist, is trained to ponder important questions of the universe such as this.  He has a theory and wrote a poem to explain it.  (See below)

Please pass it on to Rich.  I think he will enjoy it

Valley Center , Ca 

The Wombat 

With apologies to Ogden Nash 

If you ever find a wombat
Please don’t challenge it to combat
Because this weird marsupial
Has really strange poop, y’all.
Somewhere deep down in its colon
Eldritch process do roll on
Pushing, kneading, squishing, squeezing
Stretching, packing, shaping, teasing
‘Till out into the light of day
Pop little cubes of poo, hooray!
So what does Wombat do with that
Little pile where once he sat?
He stacks it up to make a wall
Then hides behind, and makes a call
To lure his prey to venture near
Not seeing that a wombat’s here.
Then out he jumps, hurling poo
At wallaby and kangaroo.
Hurls each cube with such deadly force
That it would stun a full grown horse
And worst of all are those eight corners.
If hit by one, you’d be a gorner.
So beware of cubic wombat scat;
You can’t be sure where Wombat’s at.

Lisa writes:

Hello from south Florida, 

Likely the only place in the nation without snow at the moment!

I’m a pediatric nurse practitioner and just listened to your interview with Dr. Offit. WOW! He is fabulous as always, but what a fantastic discussion about not only the COVID-19 vaccines but about vaccines in general!

My daughter was born in 2004. She became ill with rotavirus shortly before her 2nd birthday, a bit before the newer generation of rotavirus vaccines became available. When I left in the morning for work she was perfectly fine. I got a call around noon that she had been vomiting. I’m the opposite of an alarmist but by 3pm was concerned enough I left work early. I took one look at her and said, “We’re going to the ER.” She was so dehydrated she collapsed into a pile on the floor if she tried to stand up. The ER doc stopped at the doorway and, before even walking in the room or examining her said, “We’ll run some tests but I can already tell you I’m admitting her.” Treatment is of course simply fluids and time. IV fluids when necessary. Fortunately I live in a resource rich country where this is readily available. I have no doubt that, were this not true, she would have died. These resources cost me $10,000 as that was my insurance deductible at the time, but the imperfections of our healthcare system are another discussion. 

When the new rotavirus vaccine became available shortly after this, it was of course an uphill climb to obtain acceptance from providers due to the past history of the previous vaccine. However, my response was, “You’re preaching to the choir, no convincing necessary!”

Thanks for all you do to continue to share factual information!


Ann writes:


I was listening to TWiV 720 earlier in the day (technically yesterday), and am writing to add to the interesting discussion about the various Covid-19 vaccine platforms and thoughts about using a heterologous prime-boost prompted by the letter from a listener named Bill. I think it was Alan who mentioned something to the effect that there had been some discussion about setting up trials, and added, “I don’t know if anyone is going to get around to doing them.”

The National Institute for Health Research (NIHR) is supporting a trial to begin to address this issue. Recruitment started a few weeks ago. According to an article on the NIHR website, the  “’COVID-19 Heterologous Prime Boost’ study” (or “Com-Cov”) will be looking at the safety and immune response of using Oxford/AstraZeneca and Prizer/BioNTech. 

Initially the study will look at using Oxford/AstraZeneca and Pfizer/BioNTech for a heterologous primeboost (in that order and vice versa) using two different dosing intervals (4 weeks and 12 weeks) and homologous prime boost with the each of the two vaccines at both dosing intervals, which makes for eight different arms as follows:

  • Oxford/AstraZeneca and Oxford/AstraZeneca – 28 days apart
  • Oxford/AstraZeneca and Oxford/AstraZeneca – 12 weeks apart – as a control group
  • Pfizer/BioNTech and Pfizer/BioNTech – 28 days apart
  • Pfizer/BioNTech and Pfizer/BioNTech – 12 weeks apart – as a control group
  • Oxford/AstraZeneca and Pfizer/BioNTech – 28 days apart
  • Oxford/AstraZeneca and Pfizer/BioNTech – 12 weeks apart
  • Pfizer/BioNTech and Oxford/AstraZeneca – 28 days apart
  • Pfizer/BioNTech and Oxford/AstraZeneca – 12 weeks apart


It will be interesting to see what they find — whether the combinations are safe and lead to an equal or a more robust and/or more durable immune response, something Brianne had mentioned as a theoretical possibility.  Ditto for the different dosing schedules for the heterologous and the homologous prime boost regimens.  It would certainly make for more flexibility with the vaccine roll out. And, if these new combos or dosing intervals turn out to cause a reduction in response or durability, well, that’s clearly worth knowing, too. The UK plans to add other vaccines as they become licensed.

A related question for the TWiV panel: any clue when we might get some laboratory correlates of protection?

Thanks for the show!


Ps. There was also mention on episode 720 of places to get vaccine information. I would add that the New England Journal of Medicine ( has excellent Covid-19 vaccine FAQs, and share with your listeners who don’t have subscription or access through a university library that none of the SARS-CoV-2/Covid-19 articles are behind their usual paywall. 

Henry writes:

CDC guidance recently changed to allow mixing Pfizer and Moderna mRNA vaccines in “extraordinary circumstances.”  In the case of my wife, when I made her appointment using the Walgreens scheduling software, a first appointment was not available locally and I chose one an hour away in Manhattan.  The software had greater choice for the second appointment and I foolishly chose a local pharmacy.  Each of us received our first vaccine, she Pfizer in NYC, me Moderna at the local pharmacy.  We asked the two pharmacists what would happen in a month and they said that each site only offers what it has and that likely the local site would only have Moderna — and they won’t mix and match.  The scheduling software in theory allows one to reschedule but the “reschedule second dose only” button is greyed out and in any event this is all constrained by supply limits and millions of people trying for appointments. 

Both of the mRNA vaccines code for the spike protein.  I understand there are some differences between the encapsulation of the mRNA and so on, but it’s hard for me to understand why mixing and matching two vaccines that code for the same protein would be a problem.  Maybe there are some minor differences?  I ask because we may be reduced to starting over and scheduling two appointments, and then cancelling the second — but since the scheduling software doesn’t reveal which vaccine each pharmacy is offering, there’s no assurance I would be able to get a “first” dose of Pfizer.  

Sean writes:

Dear Vincent & the TWiV team,

61’F/16’C (Thanks, Kathy for that conversion mnemonic) in Las Vegas where I’m sheltering with my in-laws. 

I’m just the owner of a small improv comedy theater in New York City but an avid listener of TWiV or at least since March 2020 when we made the difficult decision to suspend all of our in-person operations. It was quite a shock businesswise but well worth it in an effort to protect the health of our staff, performers, patrons and the greater improv community. In these uncertain times, I owe you a debt of gratitude for being a trustworthy source of relevant information when I have needed it most. Your discussions, knowledge, and banter have accompanied me on countless walks and kept me sane. Thank You. 

I’ve learned so much from your podcasts that I’m the go-to guy in my family for questions about everything SARS-CoV-2. This came in handy when my wife Robin (who is an amazing singer/impressionist) asked for my help writing a character piece about Dolly Parton and the Moderna vaccine. Here is the result: 

Please feel free to share (if I got the science right). I hope we did you guys proud! 

Stay safe!



PS – My wife bought me a TWiV hoodie for Christmas that I wear as often as I can – these things are comfortable!

Ingrid writes:

Hello TWiV,

Thank you for your informative podcast, I am completely hooked!

Here’s a follow-up question to episode 720: Dr. Offit described that the goal of reaching herd immunity is to get viral transmission down to such a level so as to avoid hospitalization and death. What about those long haulers, however? I know several individuals personally who were never hospitalized, but nevertheless suffer from debilitating long-haul COVID. Surely this is something we would want to avoid moving forward?


Queens, NY

Marsha writes:

I am an avid TWIV listener. You have done us all a great service.

My question is: My husband and I have had both doses of Pfizer. We find that it is almost 2 weeks later and my husband who is 81 is very much more tired than usual. Speaking to all our senior friends that have had their 2nd dose, they are all complaining of this same feeling of exhaustion. 

Could it be related to the vaccine and our not so great immune systems.

Thanks for all your great insight and information.