"Natural immunity" is not enough

aka: You need a vaccine even if you've had COVID, especially right now


A recent CDC study summarized what most of us have been saying for a long time: You need to be vaccinated even if you’ve had COVID. Natural immunity if you’ve had COVID is not enough. I’ve seen so many people saying, “I had COVID a year ago/6 months ago/etc. I don’t need a vaccine.” You have likely heard that too. That is super risky and not correct. I’ve also seen anti-vax groups use this to say that most of us don’t need vaccines because of natural infections (based on an Israel study I talk about below). Again, that’s risky I want to talk to both groups today - those of you who are confused on whether or not you should get a vaccine after you’ve already had COVID and those of you who know anti-vax people using this as an excuse.


Immunity from natural infections is not reliable. In other words, people who get COVID do have some immunity, but the level of immunity is highly variable.

  • A recent UK study found that 1 in 4 people who had COVID did NOT go on to develop IgG antibodies (part of your immune system that is protective against future COVID infections).

  • Persons who relied on “natural immunity” were at a higher risk of hospitalizations in subsequent infections due to Delta - across all age groups (discussed below).

  • Delta now accounts for 99% of infections in the US - so, relying on past infections to protect you against Delta is really risky. Those who have had prior infections but no vaccines are at risk of hospitalizations from Delta.

Immunity from vaccines is much more reliable and produces better protection, especially during this Delta-phase. Let me show you…

The recent CDC study

They looked at adults who were hospitalized with COVID between January-September 2021 in nine states. They compared vaccinated adults with unvaccinated adults and took into account prior infections.

So, the main question is: Is natural-infection-immunity the same as vaccine-immunity? To answer this, they looked at hospitalizations associated with re-infections.

The results

The take-home message is a strikingly higher risk of re-infection (many of which were severe enough to land in the hospital) among UNVACCINATED INDIVIDUALS WITH A PRIOR INFECTION (these are called “Unvaccinated, Prior COVID” in the rest of the post).

In other words, having COVID previously did not protect enough from getting re-infected resulting in a severe form. Unvaccinated people with prior infections were 5 times more likely to have a subsequent COVID infection compared to vaccinated individuals.

To drill this down even more, I want to show you the striking risks associated with being “Unvaccinated, Prior COVID” (this means you’ve had COVID before but you’re not vaccinated). To help orient you to the Table 2 below, look at the ‘Adjusted odds ratio’ column to the far right. Let me highlight the big takeaways from this table:

The odds of hospitalization associated with re-infection for the “Unvaccinated, Prior COVID” was:

  • 5.49 times higher than fully vaccinated individuals with no previous infections

  • 7.55 times higher than fully vaccinated individuals with no previous infections during the Delta phase (WHOA!)

When we look by age group, the results are even more striking. The odds of hospitalization associated with re-infection for the “Unvaccinated, Prior COVID” was:

  • 2.57 times higher than fully vaccinated 18-64 year olds with no previous infections

  • 19.57 times higher than fully vaccinated >65 year olds with no previous infections

“What about the study from Israel that did not find this?”

A study from Israel concluded that vaccinated adults were not protected better than previous infections during the Delta phase. I see many people using this study to not get a vaccine (this one is the main study circulating in anti-vax groups too). However, that study and the CDC one differed in several important ways and likely account for the discrepancy. Here’s the discussion from the CDC study which I thought summarized the reasons well:

“These findings are consistent with evidence that neutralizing antibody titers after receipt of 2 doses of mRNA COVID-19 vaccine are high; however, these findings differ from those of a retrospective records-based cohort study in Israel,†† which did not find higher protection for vaccinated adults compared with those with previous infection during a period of Delta variant circulation. This variation is possibly related to differences in the outcome of interest and restrictions on the timing of vaccination. The Israeli cohort study assessed any positive SARS-CoV-2 test result, whereas this study examined laboratory-confirmed COVID-19 among hospitalized patients. The Israeli cohort study also only examined vaccinations that had occurred 6 months earlier, so the benefit of more recent vaccination was not examined. This report focused on the early protection from infection-induced and vaccine-induced immunity, though it is possible that estimates could be affected by time. Understanding infection-induced and vaccine-induced immunity over time is important, particularly for future studies to consider.”

Bottom line: Natural immunity is unreliable and not as good compared with vaccination - even when you take into account the Delta variant - and even among all age groups. The odds of hospitalization for re-infections among Unvaccinated, Prior COVID is higher in this Delta-phase of the pandemic (7 times higher odds) and high among all age groups (but, especially high among older adults-19 times higher odds). Yikes.

I also want to note that prior COVID+1 vaccine dose, also known as “hybrid immunity”, provides strong protection too.

So, the main takeaway is - Yes, you need a vaccine. Even after having COVID. Natural immunity is not enough.