Minor changes in glucose levels first week after COVID-19 vaccine

Patients with type 1 diabetes mellitus (T1DM) may experience a temporary change in their blood glucose levels in the first week following COVID-19 vaccination, according to a small English study.

“In a group of people with T1DM, we found that COVID-19 vaccination was associated with a temporary incremental change in interstitial glucose levels for many people [with] the effect more pronounced in individuals with better blood glucose control,” said the researchers.

The study included 97 adults (median age 44 years, 52.5 percent female) with T1DM whose interstitial glucose levels were measured using a flash glucose monitor immediately before and after receipt of the first dose of the BNT162b2 (BioNTech/Pfizer; n=45) or the ChAdOx1 nCoV-19 (Oxford/AstraZeneca; n=52) vaccine. Target range of interstitial glucose levels was 3.9–10 mmol/L.

Baseline median HbA_1c was 7.3 percent and median BMI 26.5 kg/m². All patients were on long-acting analogue insulin and prandial short-acting analogue insulin and 26 patients were also on oral hypoglycaemic therapy.

The percentage of interstitial glucose readings within target range significantly decreased in the 7 days post- vs 7 days pre-vaccination (mean 52.2 percent vs 55.0 percent; p=0.030). [Diabet Med 2021;doi:10.1111/dme.14774]

Correspondingly, the proportion of readings in the interstitial glucose categories of 10.1–13.9 and ≥14 mmol/L were elevated in the 7 days post- vs pre-vaccination (mean 27.1 percent vs 25.4 percent; p=0.085 [10.1–13.9 mmol/L] and mean 17.2 percent vs 15.1 percent; p=0.038 [≥14 mmol/L]).

Fifty-eight percent of patients with T1DM experienced a decrease in time in target range in the 7 days following COVID-19 vaccination, with 10 percent having a >20 percent reduction in time in target range, and 30 percent a >10 percent reduction in time in target range.

However, interstitial glucose variability in the 7 days after COVID-19 vaccination did not significantly differ from that 7 days before vaccination (mean 35.7 percent vs 36.3 percent; p=0.195).

“[This finding] suggests that for these people there was an overall shift upwards in interstitial glucose levels, rather than any significant change in variability,” the researchers said.

The reduction in proportion of readings in target range was still evident 2 weeks post-vaccination but to a lesser degree (mean 53.6 percent within target range).

The reduction in the proportion of readings in target range in the 7 days after COVID-19 vaccination was clearer among patients on both insulin and oral hypoglycaemic medications compared with those on insulin alone (-7.6 percent vs 2.9 percent; p=0.009).

Based on median HbA_1c, there was a 5.7 percent reduction in the time in range among those with a lower HbA_1c compared with no change among those with a higher HbA_1c (p=0.007) in the 2 weeks after vs before COVID-19 vaccination.

“[This finding] may indicate that these individuals were more sensitive to the effects of vaccination on interstitial glucose levels,” the researchers said.

Multivariate linear regression showed that estimated HbA_1c and combination treatment were independently associated with a reduction in the proportion of interstitial glucose readings in the target range (standardized beta 0.23; p=0.02 and standardized beta -0.23; p=0.036, respectively).

Age, sex, BMI, vaccine type, and duration of diabetes did not appear to affect the decrease in proportion in target range.

“On review of the clinical records, in all the individuals, there was no evidence of any other factor than the vaccination to account for the changes in interstitial glucose profile,” the researchers said. “[I]t seems likely that the observed relative hyperglycaemia was associated with the COVID-19 vaccination.”

“Our findings do indicate … that patients with T1DM should be counselled and prepared for possible transient hyperglycaemia following the COVID-19 vaccine,” they said.

They noted that research is required to assess if this effect recurs following receipt of the second dose of the COVID-19 vaccine. They also acknowledged that changes to insulin dose following vaccination were not considered.