Lib writes:

Hello TWiV team!

Lib the biostatistican here, writing from a sunny but cool 47 degrees Fahrenheit Chicago. I’m half-vaxxed (Moderna) and excited to chime in about person-days! I’m sure you’ll be hearing from several people about this, but as a recent TA for Intro to Epidemiology, and someone who’s calculated person-days for SARS-CoV-2 papers, it’s all fresh in my mind. Hopefully I can clear some things up…

First off, if we had a study with 500 people that you followed for 500 days… we can express the observed number of events in however many person-days as we want! Because just as Amy said these types of outcomes are rates, and person-days are a unit used when expressing a rate; specifically it is the denominator of a rate, and it is the number of persons multiplied by days.

So modifying the example for easier math, let’s say we followed 100 people followed for 100 days, and observed 20 events. Then we’d say the observed rate of the event in our sample was 20 per 100 x 100 person-days, or 20 out of 10,000 person-days. But we don’t have to express it out of 10,000 person-days, because it’s a rate, or in other words, a ratio! Instead, we might more naturally say the observed rate is 2 out of 1,000 person-days, or 0.2 out of 100 person-days. I’d personally use the last one, as it is the most intuitive for expressing the risk for the individual – if you were followed for 100 days like the people in the study were, you would have about a 20% chance of being positive (as 20/100 were!).

Now, you might be thinking, why don’t you just say the rate is 20 per 100, followed for 100 days? Here’s where it gets fun: first off, in the real world, you won’t get the full 100 days for every person in the study! Human research is complicated; people withdraw or are simply lost to follow-up, so the amount of time each person adds to the study will vary a great deal. By adding up the total days followed across every person, we take into account that variability. 

But even if you did manage to perfectly follow all 100 people for the 100 days, there’s another important thing to consider – when talking about an infectious disease, once someone has an event, we usually no longer consider them at risk for having the event again (aka, getting a new infection), at least within the amount of time the study is considering. This is important because the rate we want is one that only applies to the population at risk of experiencing that event. If we are talking about the event of “death,” then it’s even more clear – as Amy also said, people only die once! So usually when calculating person-days, we “censor” an individual at the time they had their event. This means that if a person joined the study and had the event on day 1, they would only add 1 person-day to the study (as well as 1 event – their “personal” rate was 1 event per 1 person-day!). 

To conclude: person-days are a unit used in time-to-event analyses. You can then get on with creating your Kaplan Meier curves and running your Cox proportional hazard models and the like, but in epidemiology, we usually want to know the overall rate of disease, and person-days is the unit for that.   

Happy to chime in about any other stats/epidemiology questions! 

<3 Lib Gray

P.S. Have to mention pregnancy: yes, if you followed 100 pregnant women from the start of pregnancy until 10 months, your ultimate rate of birth would be something in the range of 1 per 10 person-months… but it wouldn’t be exactly that, for many obvious reasons. And we would of course calculate the confidence interval!

Tony writes:

We are in the middle of world war 3,
We battle an invisible enemy,
It’s not strong or powerful on its own,
It requires stupidity to get along,
If you don’t follow the simple rules,
You are a traitor,
And collaborator,
You are a disgrace to the whole of mankind,  
Be remembered for sense, not COVIDIOT war crimes,
Too harsh you cry, no not at all,
It’s not a game you selfish fool.



Mike writes:

Hi TWiV crew,

It is 42 F and overcast in Boston, MA. I’m writing to add to the college vaccine discussion in TWiV 742. I’m a chemistry professor at one of the many Boston area colleges. We’re a small school and the pandemic has highlighted that a major part of our revenue is room and dorm fees. Therefore, our administration has been pushing to have as many students as possible taking in-person classes this past academic year. We did have twice a week testing, masking, distancing… and a little bit of hygiene theater. My school is planning for Fall 2021 to be back to normal densities of people in classes and in the dorms. I’m expecting our school and some others to require vaccinations for the fall and use that to attract students to stay in campus housing. The message would essentially be: “Everyone will be vaccinated here, if you live on campus you won’t need to distance or wear a mask.”

I love the podcast, thanks for all you do,

Mike in Boston

Dan writes:

Hello from Arlington, VA where it is a very warm 84 degrees and we are awaiting the arrival of the 17-year Cicada brood.

Thank you for all you have done to inform the public during the pandemic. It seems to me there are some difficult questions no one has yet answered about what life will look like during the next phase — when large swaths of the public are vaccinated but the virus is still circulating. 

Specifically, I’ve been wondering whether fully-vaccinated people can (a) transmit SARS-CoV-2; and (b) get “long COVID”. I haven’t really heard anyone in the media weigh in convincingly on these questions (as you know the CDC recently gave a confusing answer on the first only to reverse itself).

For me this has real-world applications as my wife and I just got vaccinated but we have two young kids (under 16) and an elderly grandparent who is vaccinated but immunocompromised.

Your thoughts on these questions, and when we might know the answers, would be greatly appreciated. Relatedly, curious to get your thoughts on when you expect the vaccine to be available for the 6-month to 11 year old age group.

Thanks again for all you do,


Monique writes:


I grew up in Montreal and now live in Calgary, Canada. I follow your podcast weekly. The vaccination in Canada is moving very slowly but it’s happening. I am a new retired junior High science teacher who started to volunteer at a  wildlife rehab clinic. I am in a process of getting immunized from Rabies.  I just received 2 out of the 3 doses. I am getting the RABAVERT vaccine. 

My question is:  can I receive my Covid vaccine at any time during my rabies immunization journey or do I have to wait a certain number of days/week before getting a :

1- The Moderna or Pfizer  vaccine 

2- Astra Zeneca vaccine 

Government of Canada is offering those 3 vaccines  and we are dependant of other countries for  vaccine supply.

Vaccination is happening in phases so people most at risk get it first. More groups are eligible as they get more doses.  We receive notification when a new group is eligible for vaccination. 

Thank you so much for keeping me engaged in learning about viruses. 


Ellen writes:

Post-covid long haulers demonstrate clearly that having the disease at all can be devastating. I do wish you’d modify your statement that the vaccines prevent severe illness and death and that’s all that matters.


6-month neurological and psychiatric outcomes in 236,379 survivors of COVID-19: a retrospective cohort study using electronic health records – The Lancet Psychiatry

Our study provides evidence for substantial neurological and psychiatric morbidity in the 6 months after COVID-19 infection. Risks were greatest in, but not limited to, patients who had severe COVID-19. This information could help in service planning and identification of research priorities. Complementary study designs, including prospective cohorts, are needed to corroborate and explain …

Sara writes:


I have been enjoying your podcasts for the last year plus!  Charles Knirsch turned me on in my role as the director of medicine at a New England boarding school and I have found them very informative and the separation of clinical highlights is greatly appreciated!

I would like to pose the following question to you:

How would you define herd immunity for COVID in a boarding school environment with a mixed day and boarding population?  Do you feel that in a closed environment that could lessen our mitigation strategies once we reach a threshold?  We have approximately 70% of our students age 16+ and I am anxiously awaiting the drop in age for the Pfizer vaccine and hopefully others to follow. 

Thank you for all you are doing, 

sara rourke


Sara Rourke, MD | Director of Medicine
Pronouns: She/Her/Hers
O’Connor Health and Wellness Center
Northfield Mount Hermon 

Abigail writes:

Below is the original email I had sent to TWIV to, amongst other things, ask for follow up on Paul Offit’s statement about long term effects of vaccines.  I had also emailed the Vaccine Education Center and they promptly sent  me this link below. Great resource and I wanted to pass it along to you in case it would be of use to others.

Keep up the good work!


From: Moser, Charlotte A

Sent: Wednesday, April 7, 2021 9:40 AM

To: Zoger, Abigail

Subject: Reply from Vaccine Education Center

Hi Abigail,

Thank you for contacting us. We wrote about this topic in a recent issue of Parents PACK, our newsletter for the public. Because it is a newsletter, we don’t cite the sources, and as you will see the events are all from different vaccine studies, so it is not part of a single paper. As such, I don’t have an easy citation to share.

Hopefully, the article will give the students information about where to look for primary articles.


Charlotte A. Moser
Assistant Director, Vaccine Education Center
Children’s Hospital of Philadelphia
Vaccines and your child: Separating fact from fiction
From: Zoger, Abigail 

Sent: Tuesday, April 6, 2021 6:49 PM


Subject: The Process of Science


As a biology instructor at a community college, I am a pandemic driven listener who has benefited greatly from listening to your podcast. (FYI, I gave my students extra credit to attend the Vaccine Town  Halls.)  After learning so much from you, listening to Episode #739 with Dr. Arturo Casadeval, I realized as a plant ecologist, I had something to offer you!

The U.C. Museum of Paleontology has a robust online education program, including Science 101, a project to help with teaching the basic process of science. They point out that science is done by testing ideas with evidence, which might be done with experiments, but also with natural experiments, comparisons and mathematical  models. Check out their interactive graphic, and this excellent discussion of different ways to test ideas:   Welcome to the world of ecologists and  evolutionary biologists!

On another note, I polled my students regarding attitudes toward vaccines and potential long-term effects of vaccines was the top concern. During episode #736, you referenced Dr. Paul Offit’s statement that long term adverse effects of vaccines all have showed up within the first 6 weeks of receiving the vaccine. I would like to pass this on to my students, but I’ve trained them to look for peer-reviewed publications to back up scientific claims. I emailed Dr. Offit to ask for citations, but am following both pathways.

Thanks for all your  hard work. I’m working my way through the Virology course from last spring and enjoying Vincent’s real time reactions to the unfolding crisis.

Abigail Zoger
Biological Sciences Department
Santa Rosa Junior College
Santa Rosa, CA

Ps: Last summer sometime, you discussed bringing on a greater diversity of scientists to the program. I hope that you pursue this avenue as  my students respond very positively to seeing scientists who look like them.  San Francisco State University has an excellent Scientist Spotlight Initiative that I have used to great effect.