Health
How coronavirus vaccines still help people who already had COVID-19
Past coronavirus infections offer some protection, but vaccines give the immune system a boost
Some people who have been infected with coronavirus have questioned whether they really need vaccines.
The U.S. Centers for Disease Control and Prevention recommends people get vaccinated regardless of whether they’ve already had COVID-19. That’s in part because it’s still unclear how long immunity lasts after an infection. Studies have shown that antibodies hang around in the blood for at least eight months after getting sick, but some recovered patients have gotten reinfected (SN: 6/11/21; SN: 8/24/20).
COVID-19 jabs give the immune systems of people who were previously infected an extra leg up to fight the coronavirus, including against new, more transmissible variants, other research shows. And because the delta variant, first identified in India, can spread among vaccinated people, that extra layer of protection for recovered patients is probably helpful (SN: 7/30/21).
“If you’ve had exposure to COVID before, don’t think you’re immune to variants,” says Benjamin Ollivere, a trauma surgeon who studies COVID-19 at the University of Nottingham in England. “Have your vaccines.”
Now, the evidence that even recovered people benefit from the shots is mounting. Based on the latest studies, here’s what experts know about past infections and getting vaccinated.
One dose may work, but two could be better
One vaccine dose might be sufficient to protect people who have already had COVID-19, lab-based studies suggest (SN: 3/3/21). One shot for those who recovered from a prior infection boosts virus-attacking antibodies to levels similar to those of vaccinated people who got two doses of an mRNA vaccine, researchers reported August 6 in JAMA. A second dose, however, didn’t further increase antibody levels for previously infected people.
Antibodies aren’t the only part of the immune response that benefit from the vaccine, although the immune proteins are crucial to prevent infection. A single shot was enough for recovered patients to reach high levels of a subset of immune cells called T cells, researchers reported August 3 in Cell Reports. T cells help coordinate and ramp up the immune response when a person is exposed to the virus again.
That suggests that people who had COVID-19 and then were vaccinated with a single dose could be as protected as fully vaccinated people who were never sick. But whether that plays out in the real world, outside the lab, is unclear.
A CDC-led study of previously infected people offers a hint. Kentucky residents who had recovered from a coronavirus infection but weren’t vaccinated were around twice as likely to get infected again as their vaccinated counterparts, researchers report August 6 in Morbidity and Mortality Weekly Report.
People who were only partially vaccinated — meaning they had received only one of two mRNA vaccine doses or had finished a vaccine regimen less than two weeks prior to being infected — were about 1.5 times as likely to be reinfected as fully vaccinated people. So, even one shot offers protection, but two doses could be slightly better. (One caveat is that few people in the study were partially vaccinated, making it harder to estimate the risk of infection.)
Larger studies will help pinpoint whether previously infected people need more than one dose for protection, the researchers say.
This story was originally published by Science News, a nonprofit independent news organization.
dk
August 22, 2021 at 2:42 PM
With respect to the new CDC case-control study: Absolute value is totally missed in this study – and is a key problem with it!
First it should be related that the background in the CDC article does not mention cohort studies already performed in this area (the cohort studies which already exist are a higher level of evidence than this type of case-control investigation). Instead the article incorrectly states “few real world epidemiologic studies exist to support the benefit of vaccination for previously infected individuals.” They should have cited NEJM Dec 23, 2020 regarding those who have COVID antibodies (no subjects had symptomatic reinfection), and the 2021 SIREN study in Lancet comparing natural immunity with vaccination, both of which address the potential protective effect after recovery from previous infection and implications for need to vaccinate them. Readers should also be aware that odds ratios — often used in these case-control studies, may exaggerate a difference between groups.
The most important key point here is that what really matters is the “absolute” risk of reinfection. Consider that the chances of an airplane crashing into your house is about 1 in a million in the countryside, but 2 in a million in the city — twice as likely. That will not cause you to move out of the city, simply because both possibilities are so rare! The newspaper headline will read: “Airplane crashes twice as likely in the city — The debate about where to live is over!” Of course that is entirely misleading, because nobody worries about such an unlikely event. So the idea here is this: if people who are previously infected without subsequent vaccination have twice the likelihood of infection as compared to vaccinated people, but the incidence in both is very, very small, then “twice as likely” does not really matter. Case-control studies like this one do not tell you anything about actual prevalence or incidence in the population, or your actual chances of getting the infection. Hence, you cannot rely on them to make public or individual health decisions. That’s one reason why case-control studies are considered a lower level of evidence than cohort studies…
Take-home message from this article: This study may suggest a slight improvement in protection from the virus with vaccination after infection, but we have no way of knowing from this study if that difference is meaningful. From the Lancet SIREN study and direct examination of the newer Cleveland Clinic “Previously Infected Individuals” study (pre-publication, but one can examine the actual data in figure 3), it appears that the difference will be insignificant.
Important comment about vaccination. People who have important risk factors such as obesity, advanced age or immunocompromise should almost certainly get vaccinated even if they were previously infected, because little data specific to their condition exists, and the above-mentioned studies looked at presumably healthy healthcare workers, and not very many people at-risk. Also we should strongly encourage most adults with neither vaccination nor prior immunity to get vaccinated.
Overall, this CDC/MMWR article is not at all a “game changer,” and does not provide useful incremental information on the topic compared with previously existing, higher level of evidence publications such as those mentioned above in New England Journal and The Lancet.
One last comment: What we really need are outcome studies with truly meaningful endpoints — such as hospitalization, endotracheal intubation and mortality — not just antibody spikes, or more positive COVID tests!
Link to Cleveland article figure: https://www.medrxiv.org/content/medrxiv/early/2021/06/05/2021.06.01.21258176/F3.large.jpg?width=800&height=600&carousel=1z
(delta variant was only present in the last two months of this Midwestern study, with low prevalence).