Podcast: COVID reinfection protection

Published March 9, 2023

The global pandemic: Three years later

Three years after we first began forecasting the trajectory of the COVID pandemic, we bring together IHME experts to discuss the ongoing effects of COVID and where we go from here.

In todays episode, Dr. Caroline Stein and Dr. Hasan Nassereldine discuss natural immunity (aka immunity from getting infected with COVID) and the best time to get a booster.



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Key takeaways:

  • Our research supports the need for annual vaccines, as protection from an infection or vaccination wanes over time. For those who have been infected, we recommend waiting six months to get a booster.
  • Natural immunity is a very good source of protection (equivalent to two doses of mRNA vaccine), but vaccines are the safest way to get immunity, especially for people at high risk.
  • Infections from a pre-Omicron variant like ancestral, Alpha, or Delta provide significantly more protection against reinfection from that same group of variants than against Omicron. 
  • Infection with an Omicron variant provides mid-level protection against a reinfection with an Omicron variant, but high protection against severe disease.

Read the research

This transcript has been lightly edited for clarity

PAULINE CHIOU: Hello and welcome to the Global Health Insights podcast at the Institute for Health Metrics and Evaluation. Im Pauline Chiou in Media Relations. The discussion topic for this podcast is natural immunity from a past COVID infection.

In other words, if you got COVID six months ago, or even 10 months ago, how protected are you? Well, we have two experts today, Dr. Caroline Stein and Dr. Hasan Nassereldine from IHME, and they just published a research paper on this very topic in The Lancet.

So thank you, both of you, for being with us.

DR. HASAN NASSERELDINE: Thank you, Pauline.     


PAULINE CHIOU: Dr. Stein, let me start with you and set the stage. You both looked at 65 different COVID studies across 19 countries. What were the key takeaways?

DR. CAROLINE STEIN: Yes. So in our systematic review and meta analysis, we look at, for all published studies, as follow-up studies, on the protection from a past infection against a subsequent COVID infection. And the main outcomes that we looked at, these results are for infection, symptomatic disease, and severe disease, which include hospitalization and death.

As the main findings that we found, we have that the protection from a pre-Omicron variant, against infection for a pre-Omicron variant is high, up to 10 months, around 80%.

However, this protection from a pre-Omicron variant, against an Omicron variant, this reduces or decreases over time to around 36% at 10 months. But at the same time, if you were infected by a pre-Omicron variant, these protections for a pre-Omicron variant are also for Omicron variants, provides you a high protection against severe disease.

So this is this is one of our main findings, that the protection against severe disease against pre-Omicron variants, as ancestral, Alpha, Delta variants, and against Omicron BA.1 variant, is high over these 10 months.

PAULINE CHIOU: Okay, so several different scenarios to digest there. Dr. Nassereldine, who exactly did you look at in these studies? For example, were they vaccinated people or unvaccinated? Had they been infected once or never infected? Can you give us a breakdown of the groups?

DR. HASAN NASSERELDINE: So most of the papers, they analyzed patients that were never infected and never vaccinated, or never infected and vaccinated, but they adjusted for the vaccination status versus people that had one infection and were never vaccinated.

And they looked at the rate at which each category or each group had a second infection. So its like a group that were never infected, vaccinated or non-vaccinated, depending on the vaccine status, the vaccine status were adjusted for, and the group that had one infection and they compared the rates of the second infection in each group.

PAULINE CHIOU: And one of the key takeaways that you came away with was natural immunity wanes over time. In other words, time duration matters. It really changes depending on the situation. So tell us a little bit more about that.

DR. HASAN NASSERELDINE: So when we were looking at how immunity wanes over time, so for the group that had an infection with a pre-Omicron variant, and for studies that also looked a second infection in pre-Omicron variant, immunity did not wane significantly.

So over a 10-month period, the protection remained over like 75% or 80%. However, when it came down to Omicron variant or the current dominant circulating variants, a pre-Omicron infection had significantly reduced protection against the current Omicron variant, so it was like 36% over a 10-month period.

So immunity waned significantly more for Omicron versus the pre-Omicron variants.

PAULINE CHIOU: Dr. Stein, as people read this research, they might come away with the idea that I got COVID, I have natural immunity, so I dont need to get a vaccine or a booster. What would you say to that?

DR. CAROLINE STEIN: This is not the case that we, so we found that, yes, a past infection provides you a protection, but we should keep in mind and weigh the risks related to this for a risk related to the co-morbidities and death. One point is that we should recognize both natural immunity and vaccination status, in consideration to obtain a full picture of an individuals immunity profile.

Vaccines continue to be super important to all of us, and mainly to protect the high-risk populations, as those that are 60 years of age, over 60 years of age and with comorbidities. Also, for those that were never infected, the vaccine is still the safest way to acquire immunity.

And also, those who were infected or received the vaccine dose more than six months ago, the vaccination is the safest way to acquire immunity against COVID.

PAULINE CHIOU: So is there some sort of magic window in terms of time interval? For example,  if youve gotten COVID and its been six months, this is now the time to get a booster?

DR. CAROLINE STEIN: Yeah, correct. It could be more than six months, in our findings, because the protection from a past infection or the protection from vaccine wanes over time. So our results, our findings, support the needs of annual vaccinations.

PAULINE CHIOU: Dr. Nassereldine, in this research, you looked at 65 studies across 19 countries, and it covered pre-Omicron variants like ancestral, Alpha, Beta, Delta, up through Omicron variants of BA.4, BA.5. Right now were dealing with XBB, so thats what the focus is on, and I realize the study didnt cover XBB, but what does all of this research mean for today, and for the future in general?

DR. HASAN NASSERELDINE: So as we have seen with the COVID virus, new variants have been emerging over the last three years. So, from our study, weve seen that protection from pre-Omicron variants, against the pre-Omicron variants, although this is only for historical purposes, it was a high level of protection.

The current dominating, circulating variants, like BA.1, BA.2, BA.4, BA.5, and so on, pre-Omicron variant had a much lower protection against those variants, and like, if a person had an infection with an Omicron variant and wanted to look at the level of protection against a second Omicron variant infection, the level of protection was okay, like around 75% for most studies.

Currently, the one circulating variant we like to think about it in the same way, like, if you currently have, if you had any pre-Omicron infection, if youre thinking about getting, what is the level of protection currently against an infection, its going to be low.

However, for severe disease, its going to be high. Also, if you got an infection with an Omicron variant and youre looking at a second Omicron variant infection, probably for severe disease, its going to be high for, against infection, is going to be kind of mid, like 75% or so.

PAULINE CHIOU: And Dr. Stein, lets talk about vaccines. The research showed that a past COVID infection, up to a certain point, can provide the same amount of protection, in terms of immunity, of at least a two-dose mRNA vaccine, particularly Moderna and Pfizer. Flesh that out for us, and how effective these vaccines are, and what other vaccines did you study?

DR. CAROLINE STEIN: Yes. So, yeah, as you say, the level and duration of the protection provided by the two doses of vaccines is at least equivalent. And these vaccines, so we have the Pfizer and Moderna, this comparison also included AstraZeneca. And we have, so these at least equivalent results of protection, against infection, symptomatic and severe disease, and for all the variants including Omicron BA.1.

We should consider that these results for vaccines are only for monovalent vaccines. So, in this case, in these comparisons, the new bivalent vaccines were not included and, and yet it is the safest way to acquire immunity, by vaccination.

PAULINE CHIOU: Dr. Nassereldine, finally, what do you want people to walk away with after learning about your research?

DR. HASAN NASSERELDINE: So I think the most important points for us from the study would be that we should recognize natural immunity as a very good source of protection, which is at least as good as a two-doses mRNA vaccine.

We should also, despite having a good protection from infection, we should also keep vaccines as the safest way to get immunity, especially for elderly people, and people with co-morbidities, and also people that work in high-risk environments.

So although we believe natural immunity is strong, we should still keep vaccination as our most important way to get immunity from COVID.

PAULINE CHIOU: Such important takeaways. Thank you so much, Dr. Hasan Nassereldine, and Dr. Caroline Stein for joining us on this podcast.

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