If you have been fully vaccinated against Covid-19, maybe you figured you no longer need to worry about contracting the coronavirus. But along with the rising number of new Covid-19 cases globally and growing concern about highly transmissible strains like the Delta variant come reports of fully vaccinated people testing positive for Covid-19.

Members of the New York Yankees, United States Olympic gymnast Kara Eaker and the United Kingdom health secretary Sajid Javid are some of those diagnosed with what is called a “breakthrough infection”.

As scary as the term may sound, the bottom line is that the existing Covid-19 vaccines are still very good at preventing symptomatic infections, and breakthrough infections happen very rarely. But just how common and how dangerous are they? Here is a guide to what you need to know.

Breakthrough infection

No vaccine is 100% effective. Dr Jonas Salk’s polio vaccine was 80%-90% effective in preventing paralytic disease. Even for the gold standard measles vaccine, the efficacy was 94% among a highly vaccinated population during large outbreaks.

Comparably, clinical trials found the mRNA vaccines from Pfizer and Moderna were 94%–95% effective at preventing symptomatic Covid-19 – much more protective than initially hoped.

A quick reminder: A vaccine efficacy of 95% does not mean that the shot protects 95% of people while the other 5% will contract the virus. Vaccine efficacy is a measure of relative risk – you need to compare a group of vaccinated people to a group of unvaccinated people under the same exposure conditions.

So consider a three-month study period during which 100 out of 10,000 unvaccinated people got Covid-19. You would expect five vaccinated people to get sick during that same time. That is 5% of the 100 unvaccinated people who fell ill, not 5% of the whole group of 10,000.

When people get infected after vaccination, scientists call these cases “breakthrough” infections because the virus broke through the protective barrier the vaccine provides.

Is it common?

Breakthrough infections are a little more frequent than previously expected and are probably increasing because of the growing dominance of the Delta variant. But infections in vaccinated people are still very rare and usually cause mild or no symptoms.

For instance, 46 US states and territories voluntarily reported 10,262 breakthrough infections to the US Centers for Disease Control and Prevention between January 1 and April 30. By comparison, there were 11.8 million Covid-19 diagnoses in total during the same period.

Beginning May 1, the Centers for Disease Control stopped monitoring vaccine breakthrough cases unless they resulted in hospitalisation or death. Through July 19, there were 5,914 patients with Covid-19 vaccine breakthrough infections who were hospitalised or died in the US, out of more than 159 million people fully vaccinated nationwide.

One study between December 15, 2020, and March 31 that included 258,716 United States veterans who received two doses of the Pfizer or Moderna vaccine, counted 410 who got breakthrough infections – that is 0.16% of the total. Similarly, a study in New York noted 86 cases of Covid-19 breakthrough infections between February 1 and April 30, among 126,367 people who were fully vaccinated, mostly with mRNA vaccines. This accounts for 1.2% of total Covid-19 cases and 0.07% of the fully vaccinated population.

Even if you are fully vaccinated, you should get tested if you have symptoms. Photo credit: Nand Kumar/ PTI

Is it serious?

The US Centers for Disease Control defines a vaccine breakthrough infection as one in which a nasal swab can detect the SARS-CoV-2 RNA or protein more than 14 days after a person has completed the full recommended doses of an FDA-authorised Covid-19 vaccine.

Note that a breakthrough infection does not necessarily mean the person feels sick – and in fact, 27% of breakthrough cases reported to the US Centers for Disease Control were asymptomatic. Only 10% of the breakthrough-infected people were known to be hospitalised (some for reasons other than Covid-19), and 2% died. For comparison, during the spring of 2020 when vaccines were not yet available, over 6% of confirmed infections were fatal.

In a study at US military treatment facilities, none of the breakthrough infections led to hospitalisation. In another study, after just one dose of Pfizer vaccine the vaccinated people who tested positive for Covid-19 had a quarter less virus in their bodies than those who were unvaccinated and tested positive.

What makes it likely?

In the United States, more than 5% of Covid-19 tests are coming back positive on average. in Alabama, Mississippi and Oklahoma, the positivity rate is above 30%. Lots of coronavirus circulating in a community pushes the chance of breakthrough infections higher.

The likelihood is greater in situations of close contacts, such as in a cramped working space, party, restaurant or stadium. Breakthrough infections are also more likely among health care workers who are in frequent contact with infected patients.

For reasons that are unclear, nationwide US Centers for Disease Control data found that women account for 63% of breakthrough infections. Some smaller studies identified women as the majority of breakthrough cases as well.

Vaccines trigger a less robust immune response among older people, and the chances of a breakthrough infection get higher with increasing age. Among the breakthrough cases tracked by the US Centers for Disease Control, 75% occurred in patients age 65 and older.

Being immunocompromised or having underlying conditions such as high blood pressure, diabetes, heart disease, chronic kidney and lung diseases and cancer increase the chances of breakthrough infections and can lead to severe Covid-19. For example, fully vaccinated organ transplant recipients were 82 times more likely to get a breakthrough infection and had a 485-fold higher risk of hospitalisation and death after a breakthrough infection compared with the vaccinated general population in one study.

Variants’ effect

Researchers developed today’s vaccines to ward off earlier strains of the SARS-CoV-2 virus. Since then new variants have emerged, many of which are better at dodging the antibodies produced by the currently authorised vaccines. While existing vaccines are still very effective against these variants for preventing hospitalization, they are less effective than against previous variants.

Two doses of the mRNA vaccines were only 79% effective at preventing symptomatic disease with Delta, compared with 89% effective in the case of the earlier Alpha variant, according to Public Health England. A single dose was only 35% protective against Delta.

About 12.5% of the 229,218 Delta variant cases across England through July 19 were among fully vaccinated people.

Israel, with high vaccination rates, has reported that full vaccination with the Pfizer vaccine might be only 39%-40.5% effective at preventing Delta variant infections of any severity, down from early estimates of 90%. Israel’s findings suggest that within six months, Covid-19 vaccines’ efficacy at preventing infection and symptomatic disease declines. The good news, though, is that the vaccine is still highly effective at protecting against hospitalisation (88%) and severe illness (91.4%) caused by the now-dominant delta variant.

Effect of vaccines

As of the end of July, 49.1% of the US population, or just over 163 million people, are fully vaccinated. Nearly 90% of Americans over the age of 65 years have received at least one dose of a vaccine.

Scientists’ models suggest that vaccination may have saved approximately 279,000 lives in the US and prevented up to 1.25 million hospitalisations by the end of June. Similarly, in England, about 30,300 deaths, 46,300 hospitalisations and 8.15 million infections may have been prevented by Covid-19 vaccines. In Israel, the high vaccination rate is thought to have caused a 77% drop in cases and a 68% drop in hospitalisations from that nation’s pandemic peak.

Across the US, only 150 out of more than 18,000 deaths due to Covid-19 in May were of people who had been fully vaccinated. That means nearly all Covid-19 deaths in the US are among those who remain unvaccinated.

The US is becoming “almost like two Americas”, as Anthony Fauci put it, divided between the vaccinated and the unvaccinated. Those who have not been fully vaccinated against Covid-19 remain at risk from the coronavirus that has so far killed more than 600,000 people in the US.

Sanjay Mishra, PhD is a Project Coordinator & Staff Scientist, Vanderbilt University Medical Center at the Vanderbilt University.

This article first appeared on The Conversation.