(The Center Square) – More than 140 people in Houston have tested positive for COVID-19 more than two weeks after receiving two doses of either Moderna or Pfizer mNRA shots.

The Houston Health Department reported that 142 people that the agency knows of who were fully vaccinated tested positive for the virus two weeks after having had the second shot, The Houston Chronicle reported.

Dr. David Persse, the city’s chief medical officer, said "breakthrough" cases are not unexpected.

“This vaccine is not dissimilar from other vaccines in that it gives you some level of protection, but no vaccine is 100 percent effective,” he said.

None of those who tested positive for COVID-19 after receiving both doses required hospitalization, Persse said.

In Wichita Falls, Texas, breakthrough cases were also reported.

Last December, after frontline workers reported testing positive for COVID-19 after having gotten the Pfizer shot, a UT Health San Antonio doctor told News4 San Antonio, “There's some people that have come down with COVID, after they got the vaccine but that's not because they got the vaccine.” Dr. Jan Patterson, infectious diseases, explained it was “because they got exposed to COVID and they were not fully immune.”

Patterson said it could take anywhere between 10 to 12 days for individuals to develop immunity after getting both doses.

Several states have reported “breakthrough cases of COVID-19” among individuals who received both doses and are considered to be “fully vaccinated.”

Washington State Department of Health officials announced it was investigating reports of more than 100 individuals who tested positive for COVID-19 more than two weeks after having received both doses of the Moderna or Pfizer or one Johnson & Johnson shot.

According to Kiro 7 CBS News affiliate, each of the positive cases were confirmed using a polymerase chain reaction test or a positive antigen test more than two weeks after the individual received their shots.

Several counties in Florida reported COVID-19 positive cases among individuals who had received the required number of doses.

Dr. Sunil Joshi, president of the Duval County Medical Foundation, compared the COVID vaccine to the flu vaccine.

“It’s like the flu shot, for instance, right. We know, we encourage people to get the flu vaccine. That doesn’t mean that you’re not going to get the flu. But the disease is significantly lessened,” Joshi told News 4 Jacksonville. “So remember, the whole goal for this, from the very beginning, has been to keep people out of the hospital. And so anything positive after the vaccine is not unusual, it can happen.”

But doctors and critics point out that the Moderna, Pfizer, and Johnson & Johnson shots do not contain SARS-2 (coronavirus) pathogens. A flu shot includes an influenza pathogen in order for the body to create antibodies to build its immune system. The COVID shots, Moderna’s mRNA1273 and BioNTech/Pfizer’s BNT162b1 and b2, are messenger mRNA genetic engineering technology developed to instruct the body to make the SARS-CoV-2 spike protein, an antigen that reportedly activates the body’s immune system to produce antibodies.

The South Carolina Department of Health and Environmental Control initially identified 134 “breakthrough cases” in Charleston among individuals who received both doses of Moderna or Pfizer shots. Other entities also reported breakthrough cases, including Roper St. Francis Healthcare and the Medical University of South Carolina.

“All of these individuals we identify who get infected or even hospitalized despite receiving two doses, that virus will be sent on to DHEC for further analysis,” infectious disease physician Dr. Kent Stock said. “The question is, is that phenomenon influencing these numbers?”

In Michigan, 246 residents tested positive for COVID-19 more than two weeks after being fully vaccinated.

Regarding breakthrough cases in California, eight physicians described the results of a study they conducted in a March 23 letter to the editor of the New England Journal of Medicine. In the study, they evaluated breakthrough cases reported by the University of California, San Diego and University of California, Los Angeles (UCLA) Health Systems.

In the UCLA optional testing program, which began on Dec. 26, 2020, for asymptomatic health care workers, researchers used a PCR nasal test in order to detect asymptomatic SARS-CoV-2 infections after receiving a COVID-19 shot.

From Dec. 16, 2020, through Feb. 9, 2021, a total of 36,659 health care workers received the first dose of a Moderna or Pfizer shot. Of them, 28,184 (77 percent) received the second dose. Among them, 379 people tested positive for COVID at least one day after receiving the shot. The majority (71 percent) tested positive within two weeks of receiving the first dose. After receiving both shots, 37 health care workers tested positive.

According to the study, “the absolute risk of testing positive for SARS-CoV-2 after vaccination was 1.19 percent among health care workers at UCSD and 0.97 percent among those at UCLA; these rates are higher than the risks reported in the trials of mRNA-1273 vaccine (Moderna) and BNT162b2 (Pfizer) vaccine.”

The report concludes, “The rarity of positive test results 14 days after administration of the second dose of vaccine is encouraging and suggests that the efficacy of these vaccines is maintained outside the trial setting. These data underscore the critical importance of continued public health mitigation measures (masking, physical distancing, daily symptom screening, and regular testing), even in environments with a high incidence of vaccination, until herd immunity is reached at large.”