Studies show Breakthrough Covid cases are rare. So, what Raises Your Risk?

Breakthrough cases of COVID-19 among vaccinated people remain rare (and are generally less serious when they do occur), despite the surge of the hypercontagious Delta variant. The attention that these cases get in the media, have put a spotlight on breakthrough cases and raised questions about the continued effectiveness of vaccines in the face of the Delta threat.

But vaccines continue to offer strong protection against the most severe forms of the disease. “We may have been giving a message that sounds like the vaccines aren’t working very well, which to me can scare the vaccinated and actually doesn’t make the unvaccinated think they should get a vaccine,” says Dr. Monica Gandhi, an infectious disease specialist and professor of medicine at the University of California, San Francisco. “[Vaccinated] people are really very protected from severe disease. … I think we need to be way more positive,” she added.

On Friday, the Centers for Disease Control and Prevention published three studies that highlight the continued effectiveness of all three approved vaccines in the U.S.: those produced by Pfizer, Moderna and Johnson & Johnson.

According to one of the studies, among approximately 600,000 COVID-19 cases recorded across 13 U.S. jurisdictions, there was some increase in breakthrough cases since the Delta variant gained dominance in the U.S. But that number remains a very small percentage of the total number of vaccinated people in the study. Of the total cases in the study, only 2,976 (a similar 8 percent) of hospitalizations, and 616 (9 percent) of deaths were reported among fully vaccinated people, compared with 34,972 (92 percent) of hospitalizations and 6,132 (91 percent) of deaths among people who were not fully vaccinated.

However, there are reports of these rare breakthrough cases, which begs the question: what increases the chances of having that happen to you?  It does appear that not everyone is at the same risk. Four things appear to contribute to how well you are protected by vaccination.

Vaccine Type

The first is the specific vaccine type you received and the relative risk reduction that each type offers. Relative risk reduction is a measure of how much a vaccine reduces the risk of someone developing COVID-19 compared to someone who didn’t get vaccinated.

Clinical trials found that the Moderna vaccine reduced a person’s risk of developing symptomatic COVID-19 by 94%, while the Pfizer vaccine reduced this risk by 95%. The Johnson&Johnson and AstraZeneca vaccines performed less well, reducing this risk by about 66% and 70% respectively (though protection offered by the AstraZeneca vaccine appeared to rise to 81% if a longer gap was left between doses).

Time Since Vaccination

But these figures don’t paint the complete picture. It’s becoming increasingly evident that length of time since vaccination is also important and is one of the reasons why the debate over booster immunizations is growing in intensity.

Early research, still in preprint (and so yet to be reviewed by other scientists), suggests that the Pfizer vaccine’s protection wanes over the six months following vaccination. Another preprint from Israel also suggests that this is the case. It’s too soon to know what happens to vaccine efficacy beyond six months in the double vaccinated, but it’s likely to reduce further.

Depends on the Variant

Another important factor is the variant of the virus that you’re facing. The reductions in risk above were calculated largely by testing vaccines against the original form of the coronavirus.

But when facing the Alpha variant, data from Public Health England suggests that two doses of the Pfizer vaccine is slightly less protective, reducing the risk of getting COVID-19 symptoms by 93%. Against Delta, the level of protection falls even further, to 88%. The AstraZeneca vaccine is also affected this way.

The COVID Symptom Study backs all of this up. Its data suggests in the two to four weeks after receiving your second Pfizer jab, you’re around 87% less likely to get COVID-19 symptoms when facing delta. After four to five months, that figure falls to 77%.

Your Immune System

It’s important to remember that the above figures refer to average risk reduction across a population. Your own risk will depend on your own levels of immunity and other person-specific factors (such as how exposed you are to the virus, which might be determined by your job).

Immune fitness typically reduces with age. Long-term medical conditions may also impair our response to vaccination. Older people or people with compromised immune systems may therefore have lower levels of vaccine-induced protection against COVID-19, or may see their protection wane more quickly.

It’s also worth remembering that the most clinically vulnerable received their vaccines first, possibly over eight months ago, which may heighten their risk of experiencing a breakthrough infection due to protection waning.

The Bottom Line

Vaccines still vastly reduce your chances of getting COVID-19. Even more important, even if you were to get a breakthrough infection, they protect against the kind of severe infection that results in hospitalization and death.

However, it’s concerning seeing breakthrough infections, and the worry is that they might increase if vaccine protection does, as suspected, fall over time. Because of this, the CDC recommends getting a booster shot this Fall.

HHS announced a plan to begin offering COVID-19 vaccine booster shots this fall. CDC’s independent advisory committee, the Advisory Committee on Immunization Practices, will continue to meet and discuss data on the evolution of the pandemic and the use of COVID-19 vaccines. ACIP will make further recommendations on the use of boosters for the public after a thorough review of the evidence.

And the U.S. is not the only one implementing boosters.  The current plan in the UK is to to give a booster dose to those most vulnerable, and is also considering whether boosters should be given more widely. Other countries, including France and Germany, are already planning on offering boosters to groups considered to be at higher risk from COVID-19. But even boosters end up being used, this shouldn’t be interpreted as vaccines not working. And in the meantime, it’s essential to promote vaccination to all those eligible who have not yet been vaccinated.

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