COVID-19: Is a 2-Dose Vaccine Mandate Justified in 2023?

by Raywat Deonandan, PhD
Epidemiologist & Associate Professor
University of Ottawa
(I add my credentials to these COVID-19 blog posts in case they get shared. I want readers to know that my opinion is supposedly an educated and informed one)

A common question I am asked is, “Are COVID-19 vaccine mandates still justified in 2023?”

There is a lot to unpack in that question.  First, were COVID vaccine mandates ever justified? That depends on your institution’s values. But strictly from an epidemiological perspective –and not a rights perspective– I believe that workplace mandates were largely justified in the pre-Omicron era, so long as accommodations were made for those unable/unwilling to comply (like mandatory testing).

Second, what would make mandates justified, again strictly from an epidemiological lens? Well, in most cases, the mandates were meant to accomplish two goals and possibly a third: (a) to slow transmission of the disease, (b) to get us closer to herd immunity, and (c) to keep people out of the hospital and morgue.

Now, herd immunity was still something we had hopes for as recently as the Spring of 2021. Back then, the Delta variant was showing its teeth, diminishing vaccine efficacy (VE) against symptomatic disease (and therefore presumably against transmission), making the achieving of herd immunity almost mathematically impossible. Omicron came along a few months later to slam the door completely on the dream of herd immunity.

So, really, any justification of vaccine mandates became mostly about the jabs’ promise of appreciably diminishing transmission. Before I tackle that question, though, we have to look at the easiest of three justifications for mandates: (c) keeping people out of the morgue and out of the hospital.

Now, I’m tempted to go through the changing VEs in the face of the many variants of SARS-CoV2 that our species has battled against during these past three years. But really, all that is pertinent to the topic of today is how effective those original jabs are against Omicron and its recent descendants. So let’s answer two questions…

(1) Do The Original Jabs Still Protect Against Hospitalization and Death?

Official surveillance data from Public Health Ontario shows that those two initial doses continue to offer significant protection against hospitalization:


Similarly, those first two jabs are still keeping people out of the morgue, though the difference in rates between the vaccinated and unvaccinated is diminishing. This might be due to waning immunity, or to COVID treatment options getting better, or to the prevailing variants being less lethal, or to the fact that almost everyone (including unvaccinated people) have protective antibodies owing to multiple reinfections, or to survivor bias (those most likely to die have already died). It’s probably a combination of all of the above, though I suspect a combination of survivor bias and infection-based immunity to be the major contributors.

One might be tempted to conclude that additional boosters (beyond the primary series plus one boost) have had a negligible effect, since the above curves seem to be converging. This would be a faulty conclusion. A study of the effectiveness of last year’s bivalent booster found that those who had received a bivalent mRNA booster vaccine dose had lower rates of hospitalization due to COVID-19 than did those who did not receive a booster, for up to 120 days after vaccination. This can be seen in the following graph, which clearly shows a lower rate of hospitalization among the boosted:

Conclusion: the initial 2 doses of COVID-19 vaccine (the co-called “primary series”) are still useful in keeping people out of the hospital, and a little useful in keeping us out of the morgue. So if your vaccine mandate policy is based on that justification, there’s certainly still an argument to be made in favour of enforcing the mandate.

(2) Do The Original Jabs Still Protect Against Transmission?

Now, before we get into this, it might be useful to revisit whether the jabs ever protected against transmission. I cover it in my 2022 post titled, “Do the Vaccines Prevent Transmission?” I revisited the issue a few months later with my post, “Does a 4th Vaccine Dose Prevent Infection?”

The answer to both questions was yes, with some important caveats that you might want to read about. Importantly, we need to understand how we define and measure transmission in vaccine science. In short, the ability of the vaccine to protect someone from becoming infected is usually given by susceptibility vaccine efficacy, or VE(s). And the its ability to prevent an infected person from infecting someone else is infectiousness vaccine efficacy, or VE(i). Some scholars would also compute a “total VE”, or VE(t), given by

VE(t)= 1- [1-(VE(s)] x [1-VE(i)]

If that means anything to you.

There also a variety of study designs that are used to estimate VE. So-called “test-negative” designs are popular and can be quite powerful when used with very large surveillance databases. Classically, the proper way is to compute “secondary attack rates”, or SARs.  A primary attack rate is the proportion of people that an infected person infects. A secondary attack rate is the proportion of people that those newly infected people then infect.

I hope you see that this is an imprecise approach, since it does not account for behaviour or circumstance. People with more active social lives or jobs that involve more human contact or people who live in crowded homes are more likely to infect others, regardless of the power of the vaccine. But in general, SARs average out these effects. So we appreciate them with some caveats.

Before I share the findings from a particularly comprehensive SAR meta-analysis, I want to draw attention to a California prison study published earlier this year. They found that having received at least one dose of any COVID vaccine reduced the probability of an infected inmate transmitting infection to his cellmate by 24%. Of course, booster doses reduced the likelihood of transmission even further.

It is important to note, though, that in the prison study, they found that for every 5 additional weeks since the last vaccine dose, breakthrough infections were 6% more likely to transmit infection to a close contact. So while the 24% number cited above is an average from all the study participants, those whose last vaccine dose was more in the distant past were much less likely to see this level of protection.

Additionally, the Oct/2022 UK Vaccine Surveillance Report found that VE(i) against the Omicron variant was 20-40% 0-3 months after the 2nd dose, and 0-30% 4-6 months after 2nd dose.

Now, back to that meta-analysis of SARs that I mentioned. They found that in the Omicron era:

VE(s) = 18.1 (-18.3 to 43.3)  and VE(i) = 18.2 (0.6 to 32.6) and VE(t) = 35.8 (13.0 to 52.6)

Those numbers in the brackets are “95% confidence intervals”. Think of them as error bars. In other words, in the face of Omicron, total VE of 2 doses is as low as 13%, but VE(s) and VE(i) are almost statistically indistinguishable from zero.

Conclusion: many months after the initial 2 doses of COVID vaccine, in the face of new variants, the ability of the vaccine to slow transmission has dropped precipitously…. but probably not to zero. Even so, if the justification for a mandate is to slow transmission, then that justification is genuinely difficult to argue now.

(3) But What About Boosters?

Now this gets tricky. For a long time, a third dose was called a “booster”. So many studies simply refer to it as such. Since then, lots of people have received multiple boosters, including last year’s bivalent booster and this season’s XBB1.5 monovalent booster. It’s really quite difficult to distinguish the effect of any one booster relative to unvaccinated people, since the comparator is usually people who have not received the most recent booster, which could often mean people with multiple shots.

Many studies, including the SAR meta-analysis mentioned above, show that booster doses (third jab) restore the vaccine’s ability to slow transmission to a significant extent.  In fact, that study shows that for booster doses in the Omicron era:

VE(s) = 40.8 (35.9 to 45.3)  and VE(i) = 32.3 (25.6 to 38.3) and VE(t) = 59.8 (54.7 to 64.5)

Look at those numbers. The lowest estimate on the scale is 25.6%.  I’ll gratefully accept a 25.6% likelihood that my infection will not be passed on. And a VE(s) lower bound of 35.9% is also nothing to sneeze at. It means almost 36% protection from infection if exposed to an active case. Not the >90% of the heyday of late 2020, but still a hearty yes, please.

As noted, we don’t yet know the VEs associated with the new XBB1.5 monovalent vaccine. But given its superior match to current circulating variants, there is every expectation that its ability to prevent symptomatic disease, and therefore transmission, will be greater than that of the prior boosters.

Conclusion: if the reason for a mandate is the slowing of transmission, then there is ample evidence to support mandating booster doses… though there’s room for discussing whether we’re talking here about solely three doses, or a more recent boost.

Of course, the application of a workplace mandate depends on legalities, ethics, values, civic engagement, and other factors beyond just vaccine efficacy. Dissenting voices must be weighed and considered. But to the extent that VE informs such policies, I feel that the evidence for effectiveness does not weigh in favour of a 2-dose mandate, but does swing in favour of booster (3rd dose) mandates.