Covid booster effectiveness wanes after 4 months, CDC study shows

Monday February 21 2022
Covid-19 vaccine.

A health worker administers a Covid-19 vaccine dose. Although protection decreases with time, a booster shot is still highly effective at preventing moderate and severe illness as well as hospitalisation. PHOTO | FILE | NMG

By Elizabeth Merab

Booster shots of the Pfizer-BioNTech and Moderna vaccines lose substantial effectiveness after about four months, but still provide significant protection against hospitalisation, a new study has shown.

In the first study on the effectiveness of third dose of mRNA vaccines, results show that immunity against severe Covid-19 disease begins to wane after four months.

“The mRNA vaccines, including the booster shot, are very effective, but effectiveness declines over time. Our findings suggest that additional doses may be necessary to maintain protection against Covid-19, especially for high-risk populations,” said study co-author Brian Dixon.

Waning immunity was observed during both the Delta and Omicron variant waves in similar fashion to how mRNA vaccine effectiveness wanes after a second dose.

Still highly effective

Although protection decreased with time, a booster shot was still highly effective at preventing moderate and severe illness as well as hospitalisation.


According to a nationwide study from the US Centers for Disease Control and Prevention (CDC), the vaccine was 91 per cent effective in preventing a vaccinated person from being hospitalised during the two months after a booster shot, the study found. But after four months, protection fell to 78 per cent.

Protection also faded from 87 to 66 per cent in preventing trips to the hospital after four months. After more than five months, vaccine effectiveness fell to roughly 31 per cent, but researchers noted that estimate was “imprecise because few data were available” for that group of people.

Although waning of vaccine protection after two doses of mRNA vaccine has been observed during the period of the Delta variant wave, little is known about durability of protection after three doses during periods of Delta or Omicron variant wave.

Last month, the World Health Organization (WHO) recommended the wider administration of a third shot of the Pfizer-BioNTech mRNA vaccine – as well as jabs for children age five and older.

In its statement, the global health body said it was recommending boosters for most adults because in the short-term, “a third dose (booster dose) may fully or partially restore vaccine effectiveness”.

In East Africa, Kenya’s Ministry of Health in December recommended booster shots to individuals six months after their initial shots. The Kenya National Immunisation Technical Advisory Group (Kenitag) recommended that fully vaccinated people be offered an additional dose of either AstraZeneca, Moderna, or Pfizer vaccine.

According to MoH daily Covid-19 immunisation situation report, 223,685 adults had received a booster dose by February 13. The government aims to administer 4.2 million booster shots by June 2022 to all eligible adults.

Protection from the two-dose vaccine regimen has declined since omicron became dominant, but a third dose boosted the immune system back up to robust levels to prevent moderately severe and severe disease.

Even so, it was not known how long the third shot’s protection lasts. Waning immunity after a third shot of mRNA vaccine during omicron has been observed in Israel and in preliminary reports from CDC, the study said.

Fourth dose

Some countries have begun offering people the fourth jab as a way of providing further protection against omicron infections, which have been found to have some level of immune escape against vaccine-derived immunity.

“Waning protection after a third vaccine dose reinforces the importance of further consideration of additional doses to sustain or improve protection against Covid-19-associated visits to emergency departments and urgent care and hospitalisations,” the US CDC study said.

Kenya’s Health Ministry has, however, said that it will not be quick to introduce a second booster (4th dose of the Covid-19 vaccine) until there is local evidence for the need.

Although some countries in the west are administering the shot to some target populations, Dr Willis Akhwale, chairperson of the National Covid-19 Vaccines Task Force, said that the Health Ministry cannot duplicate everything to justify the use of the fourth dose.

“We do not copy everything the world is doing. We generate our own data and compare it with international information which then informs our decisions,” said Dr Akhwale.

In this case, he explained that the Ministry will be considering three scenarios that will then inform its decision on whether to administer a fourth shot.

“One of the things we will be looking at is the number of people infected with Covid-19 and hospitalised after receiving the third dose,” he noted.

The Ministry in partnership with centre for epidemiological modelling and analysis (Cema) will also be comparing the status of vaccination against coverage across counties to monitor how counties with high vaccination coverage, such as Nairobi and Nyeri (43 and 44 per cent coverage, respectively), fair compared to those with low coverage such as Bomet (10 per cent) and Wajir (7.2 per cent).

“Finally, we will look at antibody levels and how they wane over time. This data will inform our decision,” said Dr Akhwale.

Dr Catherine Kyobutungi, an epidemiologist, explained that available data on the effectiveness of the fourth dose so far “is not convincing.” And if the data was there, she would only advocate for targeted third or fourth booster doses as a way of avoiding waste.

“The primary objective should be higher coverage of the whole population as fast as possible. Boosters should be secondary,” she said in an interview with The EastAfrican.