Does vaccinating people who have already had COVID-19 improve their immunity against reinfection?

Question

Bred Weinstein claimed in an interview on Triggernometry:

Why on earth are we vaccinating people who had COVID? They will get the very same immunity. There's no evidence their immunity gets better for vaccinating them. We are exposing those people to the risks of these vaccines for no benefit. It doesn't make anybody else immune. It doesn't get us any closer to herd immunity.

Is there a clinical benefit (less chance of infecting others/hospitalization/death) for vaccinating people who already have had COVID-19?

Answer 1

Assuming you're using the past tense correctly, indicating the subject has recovered from the infection, the CDC says so:

Yes, you should be vaccinated regardless of whether you already had COVID-19. That’s because experts do not yet know how long you are protected from getting sick again after recovering from COVID-19. Even if you have already recovered from COVID-19, it is possible—although rare—that you could be infected with the virus that causes COVID-19 again. Studies have shown that vaccination provides a strong boost in protection in people who have recovered from COVID-19. Learn more about why getting vaccinated is a safer way to build protection than getting infected.

If you were treated for COVID-19 with monoclonal antibodies or convalescent plasma, you should wait 90 days before getting a COVID-19 vaccine. Talk to your doctor if you are unsure what treatments you received or if you have more questions about getting a COVID-19 vaccine.

If you or your child has a history of multisystem inflammatory syndrome in adults or children (MIS-A or MIS-C), consider delaying vaccination until you or your child have recovered from being sick and for 90 days after the date of diagnosis of MIS-A or MIS-C. Learn more about the clinical considerations people with a history of multisystem MIS-C or MIS-A.

Experts are still learning more about how long vaccines protect against COVID-19. CDC will keep the public informed as new evidence becomes available.

https://www.cdc.gov/coronavirus/2019-ncov/vaccines/faq.html

The WHO appears to agree:

Take whatever vaccine is made available to you first, even if you have already had COVID-19. It is important to be vaccinated as soon as possible once it’s your turn and not wait. Approved COVID-19 vaccines provide a high degree of protection against getting seriously ill and dying from the disease, although no vaccine is 100% protective.

https://www.who.int/emergencies/diseases/novel-coronavirus-2019/covid-19-vaccines/advice

The NIH suggests the same, but mentions a caveat (now obsolete in the US, as we have more doses than people who want them):

Conclusions: Prior infection in patients with COVID-19 was highly protective against reinfection and symptomatic disease. This protection increased over time, suggesting that viral shedding or ongoing immune response may persist beyond 90 days and may not represent true reinfection. As vaccine supply is limited, patients with known history of COVID-19 could delay early vaccination to allow for the most vulnerable to access the vaccine and slow transmission.

https://pubmed.ncbi.nlm.nih.gov/33718968/

One study reported widely (but I'm unsure of its peer review status, and the sample size wasn't that big) even adds more optimism:

If you have recovered from coronavirus and confused about whether to get the vaccine or not, here is the answer. If people who recovered get even one dose of the vaccine, they are as safe as or even more than people who got 2 doses. This study has been published in Infectious Disease Journal.

On the basis of a study conducted at AIG Hospital, Hyderabad, the researchers claim that even a single dose gives a lot of protection to people who have recovered from COVID-19. The hospital has done a study on 260 health care workers. All of them had received a single dose of the Covishield vaccine between January 16 and February 5. The study was to see how much immunity the memory cells can produce when there is a disease.

The results revealed that in people who had been infected with COVID-19 before getting the vaccine, a lot of antibodies were produced in them from a single dose. Whereas in those who never had the infection, antibodies were less. Memory cells also created more immunity in such people.

https://www.dnaindia.com/india/report-one-dose-of-vaccine-enough-for-those-who-recovered-from-covid-19-study-2895310

That study was done in India, and is mostly reported in Indian-targeted news outlets.

Although the consensus is yes, a large unknown is the length of time either type of immunity will last. However, preliminary reports suggest that vaccine immunity may be better at combatting variants than natural immunity.

Answer 2

Yes, there is retrospective data that suggests that a Covid-19 vaccine administered (after some time) following an actual infection does improve clinical outcomes with respect to reinfection. According to one MMWR (CDC) study published in Aug 2021:

This report details the findings of a case-control evaluation of the association between vaccination and SARS-CoV-2 reinfection in Kentucky during May–June 2021 among persons previously infected with SARS-CoV-2 in 2020. Kentucky residents who were not vaccinated had 2.34 times the odds of reinfection compared with those who were fully vaccinated (odds ratio [OR] = 2.34; 95% confidence interval [CI] = 1.58–3.47). These findings suggest that among persons with previous SARS-CoV-2 infection, full vaccination provides additional protection against reinfection.

This brief report doesn't seem to cover any other outcome besides reinfection though.

An Israeli preprint discussed in Science in the same month has much more elaborate findings (which aren't totally germane here, but roughly: infection+vaccine is better than just vaccine in terms of immune response), but among these findings there was also

The researchers also found that people who had SARS-CoV-2 previously and received one dose of the Pfizer-BioNTech messenger RNA (mRNA) vaccine were more highly protected against reinfection than those who once had the virus and were still unvaccinated.

So in the latter regard it rather agrees with the CDC's Kentucky study. The relevant bit in the later preprint:

Model 3 examined previously infected individuals vs. previously-infected-and-once-vaccinated individuals, using “natural immunity” as the baseline group. We matched the groups in a 1:1 ratio based on age, sex and GSA. [...]

Examining previously infected individuals to those who were both previously infected and received a single dose of the vaccine, we found that the latter group had a significant 0.53-fold (95% CI, 0.3 to 0.92) (Table 4a) decreased risk for reinfection, as 20 had a positive RT-PCR test, compared to 37 in the previously infected and unvaccinated group. Symptomatic disease was present in 16 single dose vaccinees and in 23 of their unvaccinated counterparts. One COVID-19-related hospitalization occurred in the unvaccinated previously infected group. No COVID- 19-related mortality was recorded.

We conducted a further sub-analysis, compelling the single-dose vaccine to be administered after the positive RT-PCR test. This subset represented 81% of the previously-infected-and-vaccinated study group. When performing this analysis, we found a similar, though not significant, trend of decreased risk of reinfection, with an OR of 0.68 (95% CI, 0.38 to 1.21, P-value=0.188).

Some additional studies (also at preprint stage) from the US exist (and have been cited by the press) but these only compared antibody levels, as opposed to clinical outcomes as in the studies quoted above.

Vaccination boosted pre-existing levels of anti-SARS-CoV-2 spike antibodies 10-fold in previously infected individuals, but not to levels significantly higher than those of uninfected vaccinees. However, neutralizing antibody titers increased in previously infected vaccinees relative to uninfected vaccinees against every variant tested: 5.2-fold against B.1.1.7, 6.5-fold against B.1.351, 4.3-fold against P.1, and 3.4-fold against original SARS-CoV-2.

(This last study was published in April and is bit dated already as it didn't consider the Delta variant.)

So, in vaccinating those who were already infected at some point, there does appear to be at least a benefit in reducing [their] reinfection rates (approximately twofold). Other clinical outcomes haven't been quantified much in this group.

Answer 3

While not evidence of clinical benefit, there is evidence that vaccination could boost immunity against variants.

The abstract of https://science.sciencemag.org/content/early/2021/03/24/science.abg9175

Emerging SARS-CoV-2 variants have raised concerns about resistance to neutralizing antibodies elicited by previous infection or vaccination. We examined whether sera from recovered and naïve donors collected prior to, and following immunizations with existing mRNA vaccines, could neutralize the Wuhan-Hu-1 and B.1.351 variants. Pre-vaccination sera from recovered donors neutralized Wuhan-Hu-1 and sporadically neutralized B.1.351, but a single immunization boosted neutralizing titers against all variants and SARS-CoV-1 by up to 1000-fold. Neutralization was due to antibodies targeting the receptor binding domain and was not boosted by a second immunization. Immunization of naïve donors also elicited cross-neutralizing responses, but at lower titers. Our study highlights the importance of vaccinating both uninfected and previously infected persons to elicit cross-variant neutralizing antibodies.

This was linked to by this article talking about Rand Paul's claims that people who were previously infected don't need to be vaccinated. The article isn't able to directly refute the claim, but is able to offer some caveats:

First, though natural immunity appears to be very effective against the current dominant U.S. variant (known as alpha), it also appears weaker than vaccine immunity against some of the variants circulating, such as the delta variant, first detected in India. That means if those variants eventually become dominant in the U.S., people relying on natural immunity would be less protected than those who are vaccinated.

Second, there is a lack of data about whether natural immunity prevents asymptomatic transmission and infection. Several other studies, though, show vaccines do.

Third, Crotty said his studies have shown that levels of natural immunity can vary widely in individuals. His team even found a hundredfold difference in the number of immune cells among people.

“If you thought about the immune system as a basketball game and you thought about that as a team scoring 1 point, and another team scoring 100 points, that's a big difference," said Crotty. “We're not so confident that people at the low end of immunity levels would be as protected against covid-19."

But those who receive a vaccine shot have a much more consistent number of immune cells, since everyone receives the same dose amount, said Crotty.

Again these are not direct evidence of clinical benefit, but they do counter the claim that "there's no evidence their immunity gets better for vaccinating them".

Answer 4

Summary:

Two studies (SIREN from UK health care workers, the other from Israel - that's the preprint discussed briefly in Fizz' answer) comparing people who recovered (in the case of SIREN: sufficiently to still work in health care) with or without additinal vaccination against people who were not previously infected with SARS-CoV2.  

• Convalescents have at least the same (SIREN doesn't have the power to conclude more) or better immunity compared to fully vaccinated with Biontech,
• and that vaccinating these convalescents gives them still better protection (Israeli study; SIREN breakthrough numbers for vaccinated are too small to conclude anything already for unvaccinated convalescents).
• SIREN checked only against further infection as diagnosed by PCR, the Israeli study found also better protection against symptomatic infection and hospitalization.

This looks to me as:

• yes, also convalescents profit from the booster of an additional vaccination
• OTOH, those doses of vaccine would still be much better spent on someone who is neither vaccinated nor convalescent
• if the "why do we vaccinate" means why do we push convalescents to get themselves vaccinated, then from these data I don't see justification for excerting any such social or legal pressure.

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TL-DR

The Gesellschaft für Virologie ([German] virological society) has an updated statement on immunity of convalescents:

New scientific evidence on the immune response induced by SARS-CoV-2 infection or COVID-19 vaccination and observational studies on the incidence of second infections with SARS-CoV-2 allow a reassessment of the duration of immunity after having undergone SARS-CoV-2 infection.

In the early months of the pandemic, it was assumed that acquisition of SARS-CoV2 infection would result in only short-lived protective immunity. This was mainly based on the observation that certain antibody types were no longer measurable a few months after infection. However, this assessment is now outdated. A large number of studies have shown that a SARS-CoV-2 infection in humans leads to the formation of immunological memory cells, which are the actual protective mechanism of the immune system against a renewed infection1,2,3. They ensure, for example, that upon renewed contact with the pathogen, antibodies are produced very quickly that are much more effective than the antibodies that were present immediately after the first infection. In particular, they are able to neutralize variants of SARS-CoV-2 efficiently. This has been shown in convalescents who came into contact again with parts of SARS-CoV2 via vaccination4,5. Even if the antibody levels still present upon renewed viral contact are not sufficiently high to completely prevent infection with SARS-CoV-2, the rapid memory response of our immune system can at least ensure that severe courses of disease are prevented.

Meanwhile, there are also some observational studies on the protection of convalescents from re-infection. Data from several countries show that people who have undergone SARS-CoV-2 infection are very well protected against re-infection or disease, and that this protection extends to viral variants, including the delta variant6,7,8,9. In the first six months after having undergone infection, protection against re-infection with SARS-CoV-2 is at least as good as that of fully vaccinated individuals6,8. In addition, research shows that having undergone SARS-CoV-2 infection is still very protective against reinfection and severe COVID-19 disease progression even after one year8,9.

Conclusions:

• The proven duration of protection after passed SARS-CoV-2 infection is at least one year. From an immunological point of view, a significantly longer duration of protection can be assumed, which, however, has not yet been substantiated by corresponding studies due to the limited observation period.
• Based on these current findings, those who have recovered should initially be placed on the same footing as those who have been fully vaccinated for at least one year in regulations for pandemic control (e.g., mandatory testing).
• A review of the recommended timing of vaccination after surviving SARS-CoV-2 infection is advised.

 [...]

• my emphasis
• for sources and authors please see oringal text below.
• Translated with www.DeepL.com/Translator (free version) 

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Original text with sources and authors:

 Neue wissenschaftliche Erkenntnisse zur Immunantwort, die durch eine SARS-CoV-2 Infektion oder eine COVID-19 Impfung ausgelöst wird, und Beobachtungsstudien zur Häufigkeit von Zweitinfektionen mit SARS-CoV-2 erlauben eine Neubewertung der Dauer der Immunität nach durchgemachter SARS-CoV-2 Infektion.

In den ersten Monaten der Pandemie wurde davon ausgegangen, dass eine durchgemachte Infektion mit SARS-CoV2 nur eine kurzlebige schützende Immunität nach sich zieht. Dies beruhte v.a. auf der Beobachtung, dass bestimmte Antikörper-Typen bereits wenige Monate nach der Infektion nicht mehr messbar waren. Diese Einschätzung ist jedoch mittlerweile überholt. In einer Vielzahl von Untersuchungen konnte gezeigt werden, dass eine SARS-CoV-2 Infektion beim Menschen zur Ausbildung immunologischer Gedächtniszellen führt, welche der eigentliche Schutzmechanismus des Immunsystems gegen eine erneute Erkrankung sind1,2,3. Sie sorgen z.B. dafür, dass bei erneutem Kontakt mit dem Erreger sehr schnell Antikörper hergestellt werden, die wesentlich wirksamer sind als die Antikörper, welche direkt nach der ersten Infektion vorhanden waren. So sind sie insbesondere in der Lage, Varianten von SARS-CoV-2 effizient zu neutralisieren. Dies konnte bei Genesenen gezeigt werden, die über eine Impfung erneut in Kontakt mit Teilen von SARS-CoV2 kamen4,5. Selbst wenn die bei erneutem Virus-Kontakt noch vorhandenen Antikörperspiegel nicht ausreichend hoch sind, um eine Infektion mit SARS-CoV-2 komplett zu verhindern, kann die schnelle Gedächtnisantwort unseres Immunsystems zumindest dafür sorgen, dass schwere Krankheitsverläufe verhindert werden.

Mittlerweile liegen auch einige Beobachtungsstudien über den Schutz Genesener vor einer erneuten Infektion vor. Daten aus mehreren Ländern belegen, dass Menschen, die eine SARS-CoV-2 Infektion durchgemacht haben, gegen eine erneute Infektion oder Erkrankung sehr gut geschützt sind, und dass sich dieser Schutz auch auf Virusvarianten, inklusive der Delta-Variante, erstreckt6,7,8,9. In den ersten sechs Monaten nach durchgemachter Infektion ist der Schutz vor erneuter SARS-CoV-2 Infektion mindestens so gut ausgeprägt wie der Schutz von vollständig Geimpften6,8. Darüber hinaus zeigen die Untersuchungen, dass eine durchgemachte SARS-CoV-2 Infektion auch nach einem Jahr noch sehr gut vor Reinfektionen und schweren COVID-19 Krankheitsverläufen schützt8,9.

 Schlussfolgerungen:

• Die nachgewiesene Dauer des Schutzes nach durchgemachter SARS-CoV-2 Infektion beträgt mindestens ein Jahr. Aus immunologischer Sicht ist von einer deutlich längeren Schutzdauer auszugehen, die auf Grund des begrenzten Beobachtungszeitraum aber noch nicht durch entsprechende Studien belegt ist.
• Auf Grund dieser aktuellen Erkenntnisse sollten Genesene bei Regelungen zur Pandemie-Bekämpfung (z.B. Testpflicht) den vollständig Geimpften zunächst für mindestens ein Jahr gleichgestellt werden.
• Eine Überprüfung des empfohlenen Zeitpunktes einer Impfung nach überstandener SARS-CoV-2 Infektion wird angeraten.

 1 Turner et al., doi.org/10.1038/s41586-021-03647-4
[2] Ogega et al., doi.org/10.1172/JCI145516
[3] Breton et al., doi.org/10.1084/jem.20202515
[4] Reynolds et al., doi: 10.1126/science.abh1282
[5] Stamatatos et al., doi 10.1126/science.abg9175
[6] Hall et al., doi: 10.1016/S0140-6736(21)00790-X
[7] Hansen et al.,doi: 10.1016/S0140-6736(21)00575-4
[8] Gazit et al., doi.org/10.1101/2021.08.24.21262415
[9] Vitale et al., doi:10.1001/jamainternmed.2021.2959

Der Vorstand der Gesellschaft für Virologie

 Prof. Dr. Ralf Bartenschlager, Universitätsklinikum Heidelberg
Prof. Dr. Thomas Stamminger, Universitätsklinikum Ulm
Prof. Dr. Ulf Dittmer, Universitätsklinikum Essen
Prof. Dr. Sandra Ciesek, Universitätsklinikum Frankfurt
Prof. Dr. Klaus Überla, Universitätsklinikum Erlangen

Unter Beteiligung von:
PD Dr. Sebastian Ulbert
Abteilungsleiter Impfstoffe und Infektionsmodelle
Fraunhofer-Institut für Zelltherapie und Immunologie
Perlickstr. 1, 04103 Leipzig

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Study 6 (SIREN) gives the following findings:

               Total person-   Number     Incidence           [...]
               time, days      of PCR     density per
                               pos.       10 000 person-days

Negative cohort [= no history of SARS-CoV2 infection]  
Unvaccinated        442 605       902             20 
[...]
Dose 2               14 746         8              5

Positive cohort   
Unvaccinated        267 982        75              3 
[...]
Dose 2                6 232         1              2 

Study 8 (still pre-print, from Israel) concludes:

This study demonstrated that natural immunity confers longer lasting and stronger protection against infection, symptomatic disease and hospitalization caused by the Delta variant of SARS-CoV-2, compared to the BNT162b2 two-dose vaccine-induced immunity. Individuals who were both previously infected with SARS-CoV-2 and given a single dose of the vaccine gained additional protection against the Delta variant.

Answer 5

According to the study Morbidity and Mortality Weekly Report, the risk of catching Covid a second time is 2.3 times lower if you had Covid and then got vaccinated, compared to having Covid and not getting vaccinated.

I couldn't easily find how large the risk actually is. 2.34 times better than "one in five" would be excellent, 2.34 times better than "one in a million" makes little difference.